Metabolic calorimeter employing respiratory gas analysis

ABSTRACT

An indirect calorimeter for measuring the metabolic rate of a subject includes a disposable portion and a reusable portion. The disposable portion includes a respiratory connector configured to be supported in contact with the subject so as to pass inhaled and exhaled gases as the subject breathes. The disposable portion also includes a flow pathway operable to receive and pass inhaled and exhaled gases, having a first end in fluid communication with the respiratory connector and a second end in fluid communication with a source and sink for respiratory gases. The disposable portion is disposed within the reusable portion, which includes a flow meter, a temperature sensing means, a humidity sensing means, a pressure sensing means, a component gas concentration sensor, and a computation unit. The flow meter generates a signal as a function of the instantaneous flow volume of respiratory gases passing through the flow pathway and the component gas concentration sensor generates a signal as a function of the instantaneous fraction of a predetermined component gas in the exhaled gases. The computation unit receives the electrical signals from the flow meter, the temperature sensing means, the pressure sensing means, the humidity sensing means, and the concentration sensor and calculates at least one respiratory parameter for the subject as the subject breathes through the calorimeter.

REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/630,398 filed Aug. 2, 2000, which claims priority from U.S.provisional patent application Serial No. 60/146,898, filed Aug. 2,1999; No. 60/155,035, filed Sep. 20, 1999; No. 60/219,241, filed Jul.18, 2000; and No. 60/218,863, filed Jul. 18, 2000, the entire contentsof all are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates to a respiratory instrument for measuringmetabolism and related respiratory parameters by indirect calorimetryand in particular to an indirect calorimeter having a disposable portionand a reusable portion.

BACKGROUND OF THE INVENTION

[0003] U.S. Pat. Nos. 4,917,108; 5,038,792; 5,178,155; 5,179,958; and5,836,300, all to Mault, a coinventor of the present application, areincorporated herein by reference. These patents disclose systems formeasuring metabolism and related respiratory parameters through indirectcalorimetry. These instruments generally employ flow meters which passboth the inhalations and the exhalations of a user breathing through theinstrument and integrate the resulting instantaneous flow signals todetermine total full flow volumes. In one embodiment, the exhaled gasesgenerated by the user are passed through a carbon dioxide scrubberbefore passing through the flow meter so that the differences betweenthe inhaled and exhaled volumes is essentially a measurement of theoxygen consumed by the lungs. In an alternative embodiment, theconcentration of carbon dioxide exhaled by the user is determined bypassing the exhaled volume through a capnometer and integrating thatsignal with the exhaled flow volume. The oxygen consumption can then becalculated as the difference between the inhaled and exhaled volumesminus the exhaled carbon dioxide volume.

[0004] The scrubber used with certain of these systems was relativelybulky and required replenishment after extended usage. The capnometersused with the instruments to measure carbon dioxide concentration had tobe highly precise and accordingly expensive because any error inmeasurement of the carbon dioxide content of the exhalation produces asubstantially higher error in the resulting determination of the oxygencontent of the exhalation.

[0005] Additional approaches to indirect calorimetry and cardiac outputmonitoring are disclosed in Mault's co-pending application Ser. Nos.09/008,435; 09/191,782; PCT/US99/02448; PCT/US99/17553; PCT/US99/27297;PCT/US00/12745, each of which are incorporated herein by reference.

SUMMARY OF THE INVENTION

[0006] The present invention provides an indirect calorimeter formeasuring the metabolic rate of a subject. The calorimeter includes adisposable portion having a respiratory connector configured to besupported in contact with the subject so as to pass inhaled and exhaledgases as the subject breathes. The calorimeter also includes adisposable portion having a flow pathway operable to receive and passinhaled and exhaled gases. A first end of the flow pathway is in fluidcommunication with the respiratory connector and a second end is influid communication with a source and sink for respiratory gases whichmay be either the ambient atmosphere, a mechanical ventilator, or othergas mixture source. The indirect calorimeter also includes a reusableportion having a flow meter that generates electrical signals as afunction of the instantaneous flow volume of inhaled and exhaled gasespassing through the flow pathway. The reusable portion includes acomponent gas concentration sensor that generates electrical signals asa function of the instantaneous fraction of a predetermined componentgas in the inhaled and/or exhaled gases as the gases pass through theflow pathway. The reusable portion also includes a temperature sensingmeans, a humidity sensing means and an ambient pressure sensing meansthat generate an electric signal. The reusable portion further includesa computation unit that receives the electrical signals from the flowmeter, temperature sensing means, humidity sensing means, ambientpressure sensing means and the component gas concentration sensor andcalculates at least one respiratory parameter for the subject as thesubject breathes through the calorimeter.

[0007] In some embodiments, the flow pathway includes a flow tubethrough which the inhaled and exhaled gases pass and a chamber disposedbetween the first end of the pathway and the flow tube. The chambersurrounds one end of the flow tube and forms a concentric chamber.

[0008] In other embodiments, a flow tube forms part of the flow pathwayand is disposed between the two ends of the pathway. The first end ofthe pathway takes the form of an inlet conduit that extendsperpendicularly to the flow tube.

[0009] In some embodiments, the flow pathway includes an elongated flowtube through which inhalation and exhalation gases pass. The flow meteris an ultrasonic flow meter and includes two spaced apart ultrasonictransducers. The transducers are each aligned with the elongated flowtube such that ultrasonic pulses transmitted between the transducerstravel in a path that is generally parallel to the flow of fluid in theflow tube. The ultrasonic transducer may include a microscopictemperature, pressure or humidity transducer arranged in an array.

[0010] Yet other embodiments of the present invention are also disclosedin the following description and the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Other advantages and applications of the present invention willbe made apparent by the following detailed description of preferredembodiments of the invention. The description makes reference to theaccompany drawings in which:

[0012]FIG. 1 is a perspective view of a respiratory calorimeteraccording to a first embodiment of the present invention with thecalorimeter shown being used by a user;

[0013]FIG. 2 is a perspective view of the first embodiment of theinvention;

[0014]FIG. 3 is a perspective view in exploded form of the firstembodiment of the invention;

[0015]FIG. 4 is a cross sectional view of the first embodiment of theinvention, taken along lines 4-4 in FIG. 2;

[0016]FIG. 5 is a cross sectional view of the first embodiment of theinvention, taken along lines 5-5 in FIG. 4;

[0017]FIG. 6 is a perspective view in exploded form of one embodiment ofan oxygen sensor for use with the present invention;

[0018]FIG. 7 is a cross sectional view of an assembled oxygen sensor foruse with the present invention;

[0019]FIG. 8 is a perspective view of the present invention with analternative mouthpiece, shown with the disposable portion removed fromthe reusable portion;

[0020]FIG. 9 is a cross sectional view of an alternative approach toconstructing an oxygen sensor for use with the present invention;

[0021]FIG. 10 is a diagram showing the general configuration of a flowtube and ultrasonic sensors according to the present invention;

[0022]FIG. 11 is a schematic showing the electronic circuitry for usewith an embodiment of an ultrasonic flow sensing system that may be usedwith the present invention;

[0023]FIG. 12 is a schematic showing a drive signal and fluorescenceresponse signal for a fluorescence based oxygen sensor for use with thepresent invention;

[0024]FIG. 13 is a schematic showing an electronic configuration for afluorescence based oxygen sensing system for use with the presentinvention;

[0025]FIG. 14 is a schematic showing the electronic components of apreferred embodiment of the present invention; FIG. 15 is a diagramgenerally presenting a preferred approach to determination ofrespiratory parameters and calculation of metabolic rate;

[0026]FIG. 16 is a bar graph showing an example gas exchange for asingle inhalation and exhalation;

[0027]FIG. 17 is a graph showing a series of curved surfacesrepresenting the change in voltage output of the oxygen sensor withrespect to changes in the partial pressure of oxygen and an arbitrarysecond factor;

[0028]FIG. 18 is a graph showing an example of how calculated metabolicrate for a subject may change during a test;

[0029]FIG. 19 is a cross sectional view of a second embodiment of thepresent invention that is configured for improved sanitation;

[0030]FIG. 20 is a cross sectional view of a third embodiment of thepresent invention with an alternative configuration for improvedsanitation;

[0031]FIG. 21 is a perspective view in partially exploded form of arespiratory calorimeter according to the present invention and a hygienefilter module for use with the calorimeter;

[0032]FIG. 22 is a cross sectional view of the hygiene filter module ofFIG. 21;

[0033]FIG. 23 is a perspective view in partially exploded form of arespiratory calorimeter according to the present invention with analternative embodiment of a mask incorporating a hygiene barrier;

[0034]FIG. 24 is a perspective view in exploded form of the disposableportion of the mask of FIG. 23;

[0035]FIG. 25 is a perspective view in partially exploded form of arespiratory calorimeter according to the present invention with a secondalternative embodiment of a mask incorporating a hygiene barrier; and

[0036]FIG. 26 is a perspective view in exploded form of the disposableportion of the mask of FIG. 25.

DETAILED DESCRIPTION OF THE INVENTION

[0037] Basic Configuration of Calorimeter

[0038] Referring to FIGS. 1 and 2, a respiratory calorimeter accordingto the present invention is generally shown at 10. The calorimeter 10includes a body 12 and a respiratory connector, such as mask 14,extending from the body 12. In use, the body 12 is grasped in the handof a user and the mask 14 is brought into contact with the user's faceso as to surround their mouth and nose, as best shown in FIG. 1. Anoptional pair of straps 15 is also shown in FIG. 1. The straps providean alternative to holding the body 12 of the calorimeter 10 with a hand.Instead, the straps can support the mask and calorimeter in contact withthe user's face.

[0039] With the mask 14 in contact with their face, the user breathesnormally through the calorimeter 10 for a period of time. Thecalorimeter 10 measures a variety of factors and calculates one or morerespiratory parameters, such as oxygen consumption and metabolic rate. Apower button 16 is located on the top side of the calorimeter 10 andallows the user to control the calorimeter's functions. A separate lightis located below the power button 16, with the power button 16 acting asa light pipe so that the button appears illuminated when the light ison. The light is preferably used to indicate the status of thecalorimeter before, during, and after a test. A display screen isdisposed behind lens 18 on the side of the calorimeter body 12 oppositethe mask 14. Test results are displayed on the screen following a test.

[0040] Referring now to FIG. 8, a calorimeter with an alternativerespiratory connector, a mouthpiece 20 rather than the mask 14 of FIG.1, is shown. The mouthpiece 20 is preferably sized and shaped so that itmay be easily inserted into a user's mouth and respiration passesthrough it. The mouthpiece may be made from a variety of materials,including silicone. Depending on user preference, a calorimeteraccording to the present invention may be used with either a mask or amouthpiece. A mouthpiece 20 may be required for certain users, such asusers with facial hair. For accurate results, it is necessary thatsubstantially all of the user's inhalations and exhalations pass throughthe calorimeter. Therefore, when a mouthpiece 20 is used as arespiratory connector, it is preferred that a nose clip, not shown, beused to seal off the user's nostrils.

[0041] As best shown in FIG. 8, the body 12 of the calorimeterpreferably includes a disposable flow tube portion 22 and a reusablemain portion 24. The respiratory connector, such as mouthpiece 20,connects to the side of the disposable flow tube portion 22. In use,each user is given a fresh disposable portion 22 along with theappropriate respiratory connector 14 or 20. The reusable main portionmay be used with multiple users. The reusable main portion 24 has arecess 26 defined in one side and shaped so as to accept the disposableportion 22.

[0042] Basic Mechanical Configuration

[0043] Referring now to FIGS. 3 and 4, the mechanical configuration ofthe calorimeter 10 will be described in more detail. FIG. 3 illustratesall components of the calorimeter in exploded form, with the disposableportion 22 removed from the recess 26 in the main portion 24. FIG. 4 isa vertical cross section of the assembled calorimeter with thedisposable portion 22 docked in the main portion. Orientations such asvertical and horizontal are used throughout this specification. However,it should be understood that these orientation descriptors are usedmerely for convenience and are arbitrary since the calorimeter could bedescribed in other positions.

[0044] The disposable portion 22 of the calorimeter 10 is generallyelongated in the vertical direction and may be said to have a generallyvertical outward face 28 which remains exposed when the disposableportion 22 is received in the recess 26. In the preferred embodiment,the outward face has a height of about 75 mm and a width of about 28 mm.An inlet conduit 30 extends perpendicularly outwardly from this outwardface 28. In the preferred embodiment, the conduit 30 extends about 2 mmfrom the outward face 28 and has an internal diameter of about 19 mm. Aradial attachment flange 32 is provided adjacent the outer end of theinlet conduit 30 and provides for attachment of a respiratory connector,such as mask 14, as best shown in FIG. 4. The respiratory connector ispreferably securely attached and sealed to the attachment flange 32 suchas by sonic welding.

[0045] The disposable portion 22 generally consists of an outer shell 34with generally vertical side walls and a vertical flow tube 36 withinthe shell 34. The flow tube 36 is preferably cylindrical with open upperand lower ends. In the preferred embodiment, the flow tube has a lengthof about 63 mm and an internal diameter of about 12 mm. For definitionalpurposes, the flow tube 36 may be said to have an inner surface 38 onthe inside of the tube 36 and an outer surface 40 on the outside of thetube 36. Likewise, the outer shell 34 may be said to have an innersurface 42 inside the shell and an outer surface 44 outside the shell.As best shown in FIG. 4, the outer surface 40 of the flow tube 36 isspaced from the inner surface 42 of the outer shell 34 so as to define aconcentric gap between these two components of the disposable portion22. The gap varies in width somewhat at different positions around thetube. However, the gap is generally at least 5 mm in width at the top ofthe flow tube 36, with the outer surface 40 of the tube 36 and the innersurface 42 of the shell 34 drafting toward each other slightly, formolding purposes, as the gap extends downwardly.

[0046] The flow tube 36 and the outer shell 34 are interconnected by anannular flange 46 which extends between the inner surface 42 of theouter shell 34 and the outer surface 40 of the flow tube 36. The annularflange 46 interconnects the flow tube 36 and outer shell 34 and ispositioned closer to the bottom of the flow tube 36 than to the top. Inthe preferred embodiment, the flange 46 is positioned about 43 mm fromthe top of the tube 36. The flange 46 completely seals the outer surface40 of the flow tube 36 to the inner surface 42 of the outer shell 34 soas to define a concentric chamber 48 above the flange 46 and between theouter surface 40 of the flow tube 36 and inner surface 42 of the outershell 34.

[0047] As best shown in FIG. 4, the inlet conduit 30 is in fluidcommunication with the concentric chamber 48 as it intersects andpenetrates the outward face 28 of the outer shell 34 above the flange46. In the preferred embodiment, the center of the inlet conduit 30 isabout 25 mm from the top of the outward face.

[0048] Referring again to FIG. 3, the upper end of the outer shell 34 ofthe disposable 22 has a pair of sidewardly projecting, generallyhorizontal, engagement rails 50. The recess 26 in the reusable portion24 of the calorimeter has a pair of corresponding engagement slots 52,only one of which is shown. When the disposable portion 22 docks intothe recess 26 of the reusable portion 24, the engagement rails 50 slideinto the engagement slots 52 to securely interconnect the disposableportion and the remainder of the calorimeter 10. Springs 54 form part ofthe engagement slots 52 and push upwardly on the underside of theengagement rails 50. As will be clear to those of skill in the art, thedisposable portion may be made from a variety of materials. In thepreferred embodiment, the disposable is molded from ABS plastic.

[0049] According to one embodiment of the present invention, thedisposable portion 22 and reusable portion 24 are designed such thatonly specifically designed authentic disposable portions work with thereusable portion. Various approaches to accomplishing this will beapparent to those of skill in the art. For example, the disposableportion may include an authenticating device such as a chip or magneticstrip that is recognized by the reusable main portion. Preferably, thecalorimeter is operable only when an authentic disposable portion isdocked in the reusable portion. Also, the main portion may include sometype of interlock that physically “recognizes” that a correct disposableis completely docked, so that a test may not be performed with adisposable that is incorrectly or incompletely docked. As a furtheralternative, the reusable portion may recognize, record, and/or transmitsome type of identification code associated with each disposableportion. This allows accurate record keeping. Also, specific codes canbe assigned to specific users, allowing the reusable portion to identifyparticular users based on the disposable portion being docked.

[0050] Referring now to both FIGS. 3 and 4, the upper end of the recess26 in the reusable main portion 34 is defined by an upper wall 56. Theupper edge of the outer shell 34 of the disposable portion 22 fitsagainst this upper wall 56 and is held in place by the springs 54. Abottom ledge 58 generally defines the lower end of the recess 26. Thelower end of the outer shell 28 of the disposable portion 22 fitsagainst this bottom ledge 58. Therefore, the upper wall 56 of the recess26 generally seals off the upper end of the outer shell 34 of thedisposable portion 22 when the disposable portion is docked with thereusable portion. Alternatively, a seal may be provided on the upperedge of the outer shell 28 or on the upper wall 56 to improve sealing.Preferably, the sides of the disposable portion 22 also fit snuglyagainst the sides of the recess 26. It is preferred that when thedisposable portion 22 is docked into the reusable portion, very littleor no respiration gases passing through the disposable portion leaksthrough the joints between the disposable portion 22 and the remainderof the calorimeter 10.

[0051] The bottom of the recess 26 is only partially defined by thebottom ledge 58. Behind the ledge 58 is an outlet flow passage 60defined between the rear edge of the ledge 58 and the rear wall 62 ofthe recess 26.

[0052] The flow tube 36 does not extend as far, either upwardly ordownwardly, as the outer shell 34 of the disposable portion 22. Theupper end of the flow tube 36 stops short of the upper end of the outerhousing and also stops short of the upper wall 56 of the recess 26 whenthe disposable portion 22 is docked with the reusable portion. In thepreferred embodiment, a gap of about 6 mm is left between the upper endof the flow tube and the upper wall 56. Therefore, the inside of theflow tube 36 is in fluid communication with the concentric chamber 48when the disposable portion 22 is docked in the reusable portion 24. Thebottom end of the flow tube 36 also stops short of the bottom ledge 58of the recess 26. In the preferred embodiment, a gap of about 6 mm isleft between the bottom end of the flow tube and the ledge 58.Therefore, the bottom end of the flow tube 36 is not blocked off by theledge 58 and the inside of the flow tube 36 is in fluid communicationwith the outlet flow passage 60 behind the ledge 58.

[0053] Referring to both FIGS. 3 and 4, the reusable main portion 24 ofthe calorimeter 10 has an outer housing 64 constructed from multiplepieces. A semi-cylindrical main housing member 66 defines the side wallsof the reusable portion and the recess 26. A top cap 68 closes off thetop of the main housing member 66 and houses the power button 16. Aventilated bottom cap 70 closes offthe bottom of the main housing member66. The bottom cap 70 includes an open grill 72 which is in fluidcommunication with the outlet flow passage 60 within the housing.Therefore, respiration gases and atmospheric air can flow between thearea outside the calorimeter 10 and the area inside the calorimeter byflowing through the grill 72. A front cap 74 closes off the front of themain housing member 66, with front being defined as the side of thecalorimeter facing away from the mask. The front cap 74 houses the lens19 and has an oval opening 76 defined therein to allow viewing of thedisplay screen 18 behind the lens 19. As shown, the main housing member66, the top cap 68, the bottom cap 70, and the front cap 74 areinterconnected using a variety of fasteners. Alternatively, they can bedesigned so as to snap together, could be adhesively interconnected, orcould be interconnected in other ways. As will be clear to those ofskill in the art, the components forming the outer housing 64 may bemade from various materials. In the preferred embodiment, the componentsare molded from ABS plastic.

[0054] Flow Path

[0055] Referring now to FIG. 4, the flow path for respiration gasesthrough the calorimeter 10 will be described. In use, when a userexhales, their exhalation passes through the respiratory connector,through the calorimeter 10, and out to ambient air. Upon inhalation,ambient air is drawn into and through the calorimeter and through therespiratory connector to the user. This flow of respiratory gases isillustrated by arrows A-G. It should be understood that instead ofambient air, the calorimeter may be connected to a mechanicalventilator, or to a alternative gas supply.

[0056] Arrow A indicates flow to and from the user and into and out ofthe inlet conduit 30. The inlet conduit 30 interconnects with theconcentric chamber 48 so that respiration flowing from the inlet conduit30 encounters the outer surface 40 of the flow tube 36 and musttherefore turn either upwardly, downwardly, or around the sides of theouter surface 40 of the flow tube 36, as shown by arrows B. This abruptchange of flow direction has several effects. First, the lower end ofthe concentric chamber 48 acts as a saliva trap. That is, excessmoisture in a user's exhalations will tend to drop out of the exhalationflow and fall to the lower end of the concentric chamber 48. Secondly,the various routes the exhalation gas may take, and the changes ofdirection, helps to introduce turbulent flow to the flow tube. Turbulentflow through the flow tube 36 is preferred for flow measurementpurposes. Most importantly, the concentric chamber 48 serves tointroduce the respiration gases to the flow tube 36 from all radialdirections as evenly as possible. This helps to allow flow in the flowtube that can be measured linearly across a wider range of flowvelocities.

[0057] Gas flowing from the concentric chamber during exhalationencounters the upper wall 56 of the recess 26 causing the flow to turnapproximately 180°, as shown by arrows C and D, and flow downwardlythrough the inside of the flow tube 36. Flow through the flow tube 36 isindicated by arrow E. As discussed previously, the bottom end of theflow tube 36 stops short of the bottom ledge 58. Therefore, gas flowingdown the flow tube 36 during exhalation encounters the bottom ledge 58and is deflected around the ledge and into the outlet flow passage 60 asindicated by arrow F. From there, exhalation gas may pass through thegrill 72 to ambient air as indicated by arrows G.

[0058] Upon inhalation, gas flows from ambient air through the grill 72into the outlet flow passage 60 as shown by arrow G. From there, itflows around the bottom ledge 58 and into the bottom end of the flowtube 36 as indicated by arrow F. As shown, an additional concentricchamber 78 is defined between the outer surface 40 of the flow tube 36and the inner surface 42 of the outer shell 34 and below the flange 46.Upon inhalation, this concentric chamber 78 acts to create turbulence inthe flow, and to introduce gases to the flow tube from all radialpositions as evenly as possible. The inhalation gases then flow throughthe flow tube 36 as shown by arrow E and make a 180° turn as shown byarrows C and D into the concentric chamber 48. From here they flow intothe inlet conduit 30 as shown by arrow B and into the respiratoryconnector as shown by arrow A.

[0059] The above described physical configuration of the calorimeter 10takes into consideration multiple, often contradictory, factors. It ispreferred that inhalations and exhalations are not restricted as theyflow through the calorimeter. Experimentation has shown that ifinhalation and exhalation flow encounter any significant resistance,breathing becomes more difficult and the metabolic rate increases. It ispreferred that the calorimeter measure actual metabolic rate, not a rateartificially elevated by flow resistance. Flow resistance also leads toa pressure drop through the calorimeter. It is preferred that thepressure drop through the calorimeter measure less than 3 cm of water ata flow rate of one liter per second (1 L/s). As a contradictory factor,the accuracy with which flow rates through the flow tube 36 may bemeasured using an ultrasonic flow measurement system increases as flowvelocity increases. However, flow resistance increases with flowvelocity. Therefore, there is a tradeoff between flow velocitymeasurement accuracy and flow resistance. Also, a longer flow pathallows better measurement accuracy. However, increasing the flow pathlength increases the size of the calorimeter and may increase flowresistance. The above described configuration provides an excellentcombination of low flow resistance, accurate flow measurement, salivaremoval, and compact packaging.

[0060] Electronic Components

[0061] Referring now to FIGS. 3 and 4, a circuit board 88 is verticallymounted to the inside of the front cap 74 of the reusable main portion24 of the calorimeter 10. This circuit board supports or interconnectswith each of the electronic components of the calorimeter. An oxygensensor 84 is mounted to the circuit board near its lower edge andextends forwardly so that it is positioned immediately behind the rearwall 62 of the recess 26. An opening 86 in the rear wall 62 allows gasesin the flow passage 60 to contact the oxygen sensor 84. A gasket 87 ispositioned between the oxygen sensor 84 and the back of the wall 62,around the opening 86, to prevent leakage of respiration gases past theoxygen sensor. A temperature sensor 90, an ambient pressure sensor 92,and a relative humidity sensor 94 are all mounted to the circuit board88 in the positions shown. Obviously, these various sensors may belocated in other positions if desired. As will be clear to one of skillin the art, various types of sensors may be used to measure temperature,pressure, and humidity. In one embodiment of the present invention, thetemperature sensor is a thermistor, such as part number RL1005-5744-103-SA from Keystone Thermometrics, the pressure sensor is aMotorola sensor, part number MPX4115A, and the relative humidity sensoris a Honeywell sensor, part number HIH3605A. A central processing unit96 and a speaker for the calorimeter are also mounted to the circuitboard, along with an application specific integrated circuit (ASIC) 98that forms part of the ultrasonic flow sensing system. The displayscreen 18 and its associated circuitry is mounted to the front side ofthe circuit board, behind lens 19 and aligned with the hole 76 in thefront cap 74, to allow viewing of the display screen 18.

[0062] An upper ultrasonic transducer 80 is disposed in the upper wallof the recess 26 in the reusable main portion 24 of the calorimeter 10.It is connected to the circuit board 88 by wires, not shown. A lowerultrasonic transducer 82 is disposed in the bottom ledge 58 and is alsoconnected to the circuit board 88 by wires, not shown. The ultrasonictransducers 80 and 82 form part of the ultrasonic flow sensing systemand will be described in more detail hereinbelow.

[0063] In the embodiment depicted in FIGS. 3 and 4, a power supplyconnector 102 is provided on the circuit board 88 which aligns with ahole 104 in the side of the reusable portion 24 of the calorimeter 10.In this embodiment, a power cord, not shown, is connected to the powerconnector 102 and extends to a plug-in power supply for powering thecalorimeter. Alternatively, the calorimeter may include internalrechargeable or replaceable batteries in place of, or in addition to,the power connector. A communication connector 106 is also mounted tothe circuit board 88 and allows interconnection of the calorimeter 88with an external device such as a computer. This communicationsconnector may take several forms. Alternatively, or in addition, thecalorimeter may include one or more wireless communication devices, suchas an infrared (IR) transmitter and receiver, radio frequencytransceiver (Bluetooth or other), or cellular telephone or modem device.The inclusion of a wireless communication device allows the calorimeterto transmit and/or receive data to/from local/remote computing devices,including via the Internet. A cordless phone may also be incorporated inthe communication, physiological monitoring, and data processing. Thisand other approaches are disclosed in Mault's provisional patentapplication Serial No. 60/165,166, filed Nov. 12, 1999, and which isincorporated herein by reference. As a further alternative, thecalorimeter may include a slot for receiving removable memory cards.Data measured or calculated by the calorimeter may be stored on orretrieved from the removable memory card. The card may later be removedand inserted into another computing device for transfer and/or furtherprocessing of the data.

[0064] Approaches to Indirect Calorimetry

[0065] As will be clear to those of skill in the art, theabove-described calorimeter provides significant packaging, air flow,and moisture removal advantages over the prior art. As will also beclear to those of skill in the art, the actual measurements andcalculations necessary to determine various respiratory and metabolicparameters may be performed in a number of ways. A calorimeterconstructed according to the above description and accompanying Figuresmay be configured for use with several of these approaches, as will bediscussed in more detail hereinbelow. Therefore, it should be understoodthat the following description of preferred measurement and calculationapproaches are not exhaustive of the approaches possible with thephysical configuration of the calorimeter thus far described.

[0066] According to a first preferred embodiment of the presentinvention, ambient temperature, relative humidity and pressure aremeasured as well as inhalation volume and exhalation volume and oxygenconcentration. The remaining factors are either calculated or assumed asnecessary. As will be clear to those of skill in the art, each of thesefactors may be measured in a variety of ways.

[0067] Flow Sensing

[0068] According to the first preferred embodiment of the presentinvention, inhalation and exhalation volume are measured byinstantaneously measuring the flow velocity of gas through the flow tube36. Because all inhalation and exhalation passes through this tube, andthe internal diameter of the tube is known, measuring flow velocity inthe tube allows calculation of flow volume. According to the presentinvention, flow velocity in the flow tube 36 is measured using twospaced apart ultrasonic transducers.

[0069] Referring again to FIG. 4, the upper ultrasonic transducer 80 issupported in the upper wall 56 of the recess 26. The lower ultrasonictransducer 82 is supported in the bottom ledge 58 at the bottom of therecess 26. As shown, these transducers are positioned such thatultrasonic pulses traveling between the transducers 80 and 82 travelparallel to the flow in the flow tube 36 as shown by arrow E. As will beclear to those of skill in the art, transmitting ultrasonic pulses in adirection parallel to fluid flow provides advantages in measurementaccuracy.

[0070] Measurement of flow velocity using ultrasonic pulses is describedin U.S. Pat. Nos. 5,419,326; 5,503,151; 5,645,071; and 5,647,370, all toHarnoncourt et al., which are incorporated herein by reference. In theHarnoncourt patents, ultrasonic transducers are positioned so as totransmit pulses through a flowing fluid in a direction that has acomponent in the flow direction. Specifically, with fluid flowingthrough a tube, the transducers are positioned in the side walls of thetube at an angle such that ultrasonic pulses are transmitted at an angleto the fluid flow. Flow speed may be calculated based on the fact thatultrasonic pulses traveling with the flow travel faster while ultrasonicpulses traveling against the flow travel slower. Mathematicalcorrections are made for the fact that the ultrasonic pulses aretraveling at an angle to the flow. Preferably, pulses are alternatelytransmitted in a direction with the flow and in a direction against theflow so that a time difference may be calculated.

[0071] The present invention may use ultrasonic transducers comprising ametalized polymer film and a perforated metal sheet. In one preferredembodiment, the ultrasonic flow measurement system is supplied by NDD ofZurich, Switzerland and Chelmsford, Mass. The present embodimentcombines the use of ultrasonic transducers with a coaxial flow path in anovel and improved configuration.

[0072] Ultrasonic pulses are transmitted with and against the directionof flow, resulting in measurement of upstream and downstream transittimes. If the gas flow rate is zero, the transit times in eitherdirection through the gas are the same, being related to the speed ofsound and distance traveled. However, with gas flow present, theupstream transit times differ from the downstream transit times. Forconstant flow, the difference between sequential upstream and downstreamtransit times is directly related to the gas flow speed.

[0073]FIG. 10 is a simplified illustration of the general configurationused in the present embodiment. Flow rates are measured using the pairof ultrasonic transducers, 80 and 82, mounted at opposite ends of a flowpath, formed largely by flow tube 36. To send an ultrasonic pulse, ahigh voltage (approximately 200 V) is applied to one transducer, say 80,and the voltage is then quickly removed. This causes transducer 80 toresonate at its natural frequency and to function as an acoustictransmitter. A voltage of approximately 100 V is applied to the othertransducer 82, enabling it to act as an acoustic receiver (or acousticdetector). The DC bias must be applied to the receiving transducer 82 inorder for it to generate the maximum electrical signal. The transit timeis the time between the transmission of the pulse from transducer 80 anddetection of the pulse by transducer 82. The roles of transmitter anddetector are then reversed, in order to measure a transit time for apulse traveling in the opposite direction.

[0074] A series of transit time measurements of the formU1-D1-U2-D2-U3-D3 are hence obtained, where U and D refer to transittimes for pulses traveling up or down the flow tube, respectively, andthe numbers refer to the sequence of measurement. (The terms up and downare appropriate for the configuration shown in FIG. 10; however in otherembodiments the flow orientation may be horizontal, oblique, etc.) Byaveraging U1 and U2, we obtain an estimated uptime at the time D1 wasmeasured by linear interpolation. To obtain a transit time difference,and hence flow rate, at the time that D1 was measured, we compare D1with the average of U1 and U2. Similarly, to obtain a flow rate at thetime U2 was measured, we compare U2 with the average of D1 and D2. Thisis but one simple method of processing the measured data. Otherapproaches will be clear to those of skill in the art.

[0075] A schematic of the electronic drive scheme is shown in FIG. 11.Ultrasonic transducers 80 and 82 are preferably controlled by an ASIC(application-specific integrated circuit) 98, using transducer controlcircuitry 110. The ASIC 98 is used to control the transmission anddetection of ultrasonic pulses, and communicates with the CPU (centralprocessing unit) 96 of the calorimeter using a serial UART (universalasynchronous receiver transmitter) operating at 19.2 Kbaud. Aconventional boost converter 112, regulated by the ASIC 98, is used togenerate a high voltage in the range 190-230 V (DC) from the low voltage(5 V) supply 114. The high voltages are required to operate theultrasonic transducers. The low voltage supply 114 also powers otherdevice elements. Other electronic control schemes with similarfunctionality may be used.

[0076] A command is sent from the CPU 96 to the ASIC 98 to start theflow measurements. The ASIC, through control circuitry 110, applies 200V to one transducer (say 80). This voltage is then discharged, causingan approximately 35 kHz (resonant frequency of the transducer) pulse tobe emitted. At the same time, 100 V is applied to the other transducer82. A 10 MHz clock within the ASIC 98, controlled by a crystal 116associated with the ASIC, drives a 100 MHz counter that counts in 10 nsincrements starting from the time the pulse is sent. When a 35 kHzsignal is received from the detecting transducer 82, the count isstopped, and the transit time value (in the form of a number ‘N’ of 10ns time intervals) is sent to the CPU 96 using the serial connection.Every 5 ms, the acoustic transmitter and acoustic receiver switch roles,so that an ultrasonic pulse is then transmitted in the oppositedirection along the flow path.

[0077] A typical transit time in the present embodiment is 220 μs, or2200×10 ns time intervals, in which case the number 2200 would be sentto the CPU as a data byte. Transit time data are sent from the ASIC tothe CPU over the UART. An interrupt service routine is used to capturethe serial bytes as they are received by the CPU.

[0078] Transit times for pulses traveling up and down the flow tube(up-times and down-times) and the difference between sequentialmeasurements are stored in three separate buffers. The difference bufferis used to zero the device, so that the flow reading is zero for noactual flow. The difference buffer is also used to detect inhale,exhale, and no-flow states. There are additional instrumental delaytimes in the transit time measurement process, typically approximately20 μs. These may differ for up-time and down-time measurements, and canbe compensated for by subtracting the delay from the transit time data.

[0079] A software process is used to calculate the flow values, usingthe method of averaging e.g. two up-times and comparing with theintervening down-time (as discussed in more detail earlier). The flowvalues are combined with the cross-sectional area of the tube (113 mm²in the presently preferred embodiment), path length (distance betweenthe transducers, 76 mm in the presently preferred embodiment), andcalibration factors for up-flow and down-flow to obtain flow rates aswell as linearization constants. The flow rates are summed overexhalation or inhalation periods to obtain flow volumes.

[0080] In the present preferred embodiment, the interrupt serviceroutine of the CPU is used to form cumulative sums of up-times anddown-times for storage in separate buffers. The software process samplesthese periodically, e.g. every 100 ms (20 samples). This effectivelyaverages the flow rate measurements, leading to higher resolution in thecalculated flow volumes. In the present preferred embodiment, theresolution in measured flow volume is approximately 0.9 ml/s perindividual measurement, or approximately 0.045 ml/s for an average of 20readings.

[0081] Other embodiments are possible. For example, ultrasonic flowsensors may be obtained from other sources. Some sensors use thesing-around method of flow rate determination. Ultrasonic pulses aretransmitted along the flow path from one transducer to the other, a newpulse being sent once the previous pulse has been received. Thefrequency of pulse sending is related to the transit time along thetube. The role of transmitter and detector are reversed after some time,or some number of pulses, and a train of pulses is sent along the flowpath in the opposite direction, a new pulse being sent once the previouspulse is detected. A new frequency of pulse sending is measured. Hencethe equivalent of up and down times are determined from the frequencymeasurements, and can be treated as described above.

[0082] Micromachined ultrasonic transducer arrays are available fromSensant of San Jose, Calif. These sensors have the advantage of lownoise, high frequency range, potentially lower drive voltages, and haveadvantages for use in the present invention. For example, pulserepetition rates may be higher, allowing instantaneous flow rates to bemeasured more frequently (i.e. with higher resolution), giving moreaccurate integrated flow volumes. Micromachined temperature, pressure,and humidity sensors may be integrated into the ultrasonic arrays,allowing the effects of these environmental factors on ultrasonictransducer performance to be compensated. For example, distortion ofmicromachined structures due to environmental effects may be monitoredusing electric capacitance. Using an array, or a number of arrays,transit time variations over the lateral dimension (perpendicular toflow direction) of the flow tube may be measured (cross-sectional flowimaging) and integrated. Different sensors on the array may be used astransmitters and detectors at the same time, allowing upstream anddownstream transit times to be measured simultaneously, so thataveraging methods are not required.

[0083] As will be clear to those of skill in the art, other approachesto flow sensing may also be used in place of, or in addition to, theultrasonic flow sensing on the preferred embodiment. For example, flowrates may be determined using tiny impellers in the flow path, hot wirebased mass flow meters, and pressure differential type flow meters. Aswill be clear to those of skill in the art, the present preferredembodiment could be adapted to use these or other approaches to flowmeasurement.

[0084] Oxygen Sensor

[0085] As mentioned previously, the oxygen concentration of theexhalation flow is also measured in the present invention. Specifically,instantaneous oxygen concentration is measured at the same time as flowis measured. By “instantaneous” it is meant that the oxygen sensing hasa very fast response time. Preferably, the response time of an oxygensensor for use with the present invention is 100 msec or less. In someembodiments, the response time is 30-40 msec or less.

[0086] Oxygen concentration may be measured in a variety of ways. In thepresently preferred embodiment of the present invention, afluorescence-based oxygen sensor is used to determine the partialpressure of oxygen in the exhalation.

[0087] As best shown in FIGS. 4 and 5, the oxygen sensor 84 is mountedadjacent a window 86 in the back wall 62 of the recess 26. This placesthe oxygen sensor 84 in contact with the inhalation and exhalation gasespassing through the outlet flow passage 60. This positioning alsoexposes the oxygen sensor to a turbulent flow of gas, which ispreferred.

[0088] Fluorescence based oxygen sensors are known in the art, forexample as described by Colvin (U.S. Pat. Nos. 5,517,313; 5,894,351;5,910,661; and 5,917,605; and PCT International Publication WO 00/13003,all of which are incorporated herein by reference). A sensor typicallycomprises an oxygen permeable film in which oxygen-indicatingfluorescent molecules are embedded. In U.S. Pat. Nos. 5,517,313 and5,894,351, Colvin describes sensors using a silicone polymer film, andsuggests using a ruthenium complex,tris(4,7-diphenyl-1,10-phenanthroline)ruthenium (II) perchlorate, as theoxygen indicator fluorophore molecule. The orange-red fluorescence ofthis ruthenium complex is quenched by the local presence of oxygen.Oxygen diffuses into the oxygen permeable film from the gas flowing overthe film, inducing fluorescence quenching. The time response of thequenching effect, relative to concentration changes of oxygen in the gasoutside the film, is related to the thickness of the film. Thin filmsare preferred for a rapid response, as described in U.S. Pat. No.5,517,313.

[0089] Referring now to FIGS. 6 and 7, the fluorescence based oxygensensor used in the present embodiment is shown generally at 120. FIG. 6is an exploded view and FIG. 7 is a cross sectional view. The presentlypreferred sensor is supplied by Sensors for Medicine and Science, Inc.,based on the technology described in the Colvin patents. A circuit board144 has a plurality of pins 149 extending downwardly for interconnectingthe sensor 120, both mechanically and electrically, with the maincircuit board 88 in the calorimeter. An LED 132 is mounted generally tothe center of the top of the circuit board. A pair of photodiodes 134and 136 are also mounted to the top of the circuit board 144. Thephotodiodes 134 and 136 are mounted symmetrically on opposite sides of,and a short distance from, the LED 132. An optical filter is mounted ontop of each photodiode; filter 138 is mounted on photodiode 134 andfilter 140 is mounted on photodiode 136. The optical filters are bondedto the photodiodes with an optically clear adhesive.

[0090] A heat spreader 142, preferably a thin copper sheet withdowntumed edges, is mounted to the top of the circuit board. The heatspreader has a downwardly extending foot 143 at each of its fourcorners, each of which engage a hole 145 in the circuit board 144. Thefeet 143 and the downtumed edges of the heat spreader 142 support thecentral portion of the heat spreader 142 a short distance above thecircuit board 144, leaving a gap therebetween. The LED 132, thephotodiodes 134 and 136, and the filters 138 and 140 are disposed inthis gap between the circuit board 144 and the heat spreader 142. Tworound holes 146 are cut in the heat spreader, one hole being directlyabove each of the photodiodes 134 and 136. Two pieces of glass substrate128 and 130 are mounted to the top of the heat spreader 142, with onepiece being mounted directly on top of each of the holes 146. As shown,these pieces of substrate 128 and 130 are square. A circle offluorescent film is formed on top of each of the pieces of substrate;film circle 122 is formed on substrate 128 and film circle 124 is formedon substrate 130. A gas impermeable glass cover 126 is disposed overfilm circle 124 and bonded to the glass substrate 130 with epoxy 125.Therefore, film circle 124 is sealed in by the cover 126 above and theepoxy 125 at the edges. This results in one of the film circles, 122,being exposed to the surrounding atmosphere, while the other filmcircle, 124, is sealed in and not exposed. Therefore, film circle 124does not react to changes in oxygen concentration while film circle 122does. Film circle 122 will be referred to as a sensing region and filmcircle 124 will be referred to as a reference region.

[0091] Referring again to FIG. 7, the gap between the circuit board 144and the heat spreader 142, as well as the holes 146, are filled with anoptically clear waveguide material 141. The waveguide material 141serves to optically couple the LED 132 to the glass substrates 128 and130, making the substrates an integral part of the waveguide. Thewaveguide material also optically couples the sensing region 122 andreference region 124 to the filters 138 and 140 and the photodiodes 134and 136. The result is a continuous optical waveguide that opticallycouples these components. Suitable waveguide materials are manufacturedby Norland Products of New Brunswick, N.J., and by Epoxy Technology ofBilerica, Mass., the latter under the name EPOTEK®.

[0092] In order to avoid problems with condensation forming on thesensing region 122 and the reference region 124, the regions arepreferably both warmed using the heat spreader 142. For this purpose,small heaters 148, comprising resistors, are mounted to the circuitboard 144 adjacent each of the foot mounting holes 145. The heatspreader feet 143 are soldered into the holes 145, and to the heaters148 so that heat is transferred into the spreader. A thermistor 147 ismounted to the circuit board 144 in a position such that it contacts oneof the downtumed edges of the heat spreader 142 when the sensor isassembled. The thermistor 147 may be soldered to the edge to improveheat transfer. The thermistor 147 is then used to monitor thetemperature of the heat spreader 142, and the heaters 148 are controlledso as to maintain a generally constant temperature. An EEPROM 155,containing calibration data for the oxygen sensor, is mounted to theunderside of the circuit board 144.

[0093] The fluorescent films 122 and 124 are formed by an oxygenpermeable film containing oxygen-indicating fluorescent molecules, suchas ruthenium complexes. In the presently preferred embodiment, theoxygen permeable films are a porous glass, such as sol-gel.

[0094] Radiation from the LED 132, preferably a blue light-emittingdiode (LED), is transmitted to the sensing region 122 and the referenceregion 124 by the optical waveguide material 141. The wavelengthemission of the LED 132 is chosen to induce fluorescence from thefluorescent film regions 122 and 124; other wavelengths may be used withother fluorophores. Orange-red fluorescence emissions from sensing andreference regions are detected by the two photodiodes. Photodiode 134detects fluorescence from the reference region 124, and photodiode 136detects fluorescence from the sensing region 122. The photodiode outputsare fed into high-speed transconductance amplifiers, as described below.The optical filters 138 and 140 overlie the photodiodes, to pass theorange-red fluorescence radiation while rejecting other wavelengths, inparticular blue radiation from the LED. The optical filters 138 and 140may be made an epoxy coating, a glass filter, or a polymeric-based sheetmaterial. Preferably, a prefabricated polymeric-based sheet material isused. The emissions from the LED 132 and the fluorescence emissions fromthe films 122 and 124 pass through holes 146 in the plate 142.Preferably, the film circles 122 and 124, the holes 146, and the activeareas of the photodiodes 134 and 136 are all circles of similardiameter.

[0095] During oxygen sensing measurements, the substrates 128 and 130and sensing region 122 and reference region 124 are maintained atapproximately 45° C. to reduce problems associated with moisturecondensation. The heating of the substrate is achieved by passingelectrical current through the four surface-mounted resistors 148. Thetemperature of the copper plate 142 is monitored by the thermistor 147,allowing the heating current through the resistors and temperature to beregulated. If moisture was eliminated from the gas flow by some means,e.g. chemical drying, water absorbing/adsorbing substances, membranes,filters, foam sheets, etc., or prevented from condensing on thefluorescent film, such as by some surface treatment (an oxygen-permeablehydrophobic film or other approaches), then the oxygen sensor need notbe heated. Temperature stability is improved by heating, however theoxygen sensitivity is better at lower temperatures.

[0096] The radiation output of the LED is preferably modulated using anelectrically modulated drive current. A modulation frequency of 2 kHz isused in the presently preferred embodiment. Other modulationfrequencies, such as 1-10 kHz, may be used. The present embodimentdetermines oxygen partial pressure based on fluorescent intensitymeasurements, in which the decrease in fluorescence intensity due tooxygen quenching is detected. The less oxygen that is present, the morefluorescence that will be detected, the more oxygen, the lessfluorescence. Each time the LED 132 is illuminated, there is afluorescence response, the intensity of which varies depending on theamount of oxygen that is present. As known to those of skill in the art,the fluorescence response is not instantaneous, but rather there is alag before the fluorescent material fluoresces in response toillumination by the LED 132. Likewise, there is a lag between the timethe LED 32 is turned off and the time that fluorescence stops. This isknown as decay time. Preferably, the time period of the appliedmodulation is chosen to be significantly greater than the fluorescencedecay time. FIG. 12 shows schematically an example of a possible appliedradiation intensity vs. time signal (the squarewave signal 150), alongwith a possible fluorescent response signal (the rounded signal 152). Inthe current embodiment, the time period of the squarewave is of theorder of 0.50 msec, whereas the fluorescence decay time is on the orderof 3 μsec (0.003 msec). An alternative approach to determining oxygenconcentration is based on detecting changes in the fluorescence decaytime, using e.g. measurements of the phase delay of a fluorescencesignal relative to the excitation signal. For example, in FIG. 12, thephase delay of the fluorescence decay may be measured so as tocorrespond to the period shown as “a”. This phase delay varies withoxygen concentration and may therefore be used as an indicator of oxygenconcentration. In such fluorescence decay measurements, highermodulation frequencies may be used. The intensity modulation of the LEDoutput may also be sinusoidal. The thickness and porosity of thefluorescent films may also be adjusted to control the diffusion-limitedresponse time of the fluorescence signals.

[0097] Signals from the photodiode 136 (the sensing region signal) andphotodiode 134 (the reference region signal) are passed through similaramplifying, filtering, and demodulation stages to obtain DC signalscorresponding to the fluorescence intensity from both regions. If thereference signal and the sensing signal are of different levels at zerooxygen concentration, their respective gains in the amplifier stage maybe adjusted to compensate. After amplification and conversion to DC, thesensing region signal and the reference region signal are compared toobtain a signal that is theoretically independent of error sources suchas temperature change, LED intensity changes, etc. In the presentembodiment, this comparison takes the form of:

[0098] Signal=Sensing Region−K*(Reference Region−Reference Baseline)

[0099] where K is an experimentally determined constant. Alternatively,the two signals may be ratioed.

[0100]FIG. 13 shows a schematic diagram of the processing of the signalfrom photodiode 136. The signal enters at 156, is passed through a highpass filter 158 to remove DC and low frequency ambient light producedsignals (e.g. low frequency stray light from electric lamps), and ispassed to an inverting AC amplifying stage 160. The AC signal is passedthrough another high-pass filter 162 into another amplifying stage 164.The amplified AC signal is then demodulated using an analog switch 166(based on an Analog Devices chip ADG719BRM, though other devices may beused). This switch alternates between the signal when the LED is on, anda constant reference voltage (used as the virtual ground for theamplifying stages, shown input at 168) when the LED is off. Thefollowing amplifying stage 170 alternates between a gain of 0.5 and−0.5, demodulating the signal. The signal is then passed through afurther amplifying stage 172 and a low pass filter 174. The use of sucha scheme, sometimes termed a synchronous amplifier, or lock-inamplifier, considerably improves the signal to noise ratio. This is aconventional technique, and other schemes may be used.

[0101] To compute the actual signal due to the oxygen sensor, a baselinemeasurement is first made with the LED turned off. The signal obtainedwith the LED turned on (and modulated) is read and subtracted from thebaseline to find the final oxygen sensor signal.

[0102] Electronic Circuitry and Components

[0103]FIG. 14 shows a simplified schematic of the calorimeter, in termsof its electrical configuration. The calorimeter has a centralprocessing unit (CPU) 96 which controls the overall operation of thedevice.

[0104] The oxygen sensor, shown generally at 120, comprises a blue LED132, a fluorescence quenching oxygen sensing region 122, a fluorescentreference region 124, a thermistor 147, a heater 148, and an EEPROM 155containing calibration data for the sensor 120. The LED 132 receives amodulated drive current, controlled by oscillator/modulator 153.

[0105] The ultrasonic transducers 80 and 82 are controlled by the ASIC98, using control circuitry 110 to direct signals and high voltage fromhigh voltage source 112 to the sensors, and to pass detected ultrasonicpulses to the ASIC 98. The ASIC 98 and CPU 96 are connected by a serialUART.

[0106] When the device is turned on, by pressing the switch 178, the CPUdirects the heater 148 to warm the fluorescent regions to approximately45° C. An indicator light 176 shows a warming up state. The temperatureof the oxygen sensor is monitored by the thermistor 147. During thisperiod, the unit calibrates the oxygen sensor and a zero-flow test isperformed, as explained later. When the sensor temperature isstabilized, as determined by the CPU from thermistor readings, the light176 indicates that the device is ready to use.

[0107] Once the device is ready to start the breath analysis, the personbreaths through the device, and the flows of inhaled air and exhaled gasare monitored by the ultrasonic transducers 80 and 82. Flow through theunit triggers data recording. Flow volumes are calculated by the CPUfrom serial data received from the ASIC. The ASIC determines the timebetween sending an ultrasonic pulse from one transducer, and receivingit using the other.

[0108] The oxygen sensor provides two electrical signals fromphotodiodes 134 and 136, both modulated at the same frequency as the LED132. The signal from photodiode 134, due to fluorescence from thereference region 124, is independent of oxygen partial pressure in thegas flowing over the reference region. The signal from photodiode 136,due to the oxygen sensing region 122, is reduced in intensity (quenched)by the presence of oxygen at the sensing region. The signals from thetwo photodiodes are passed through similar filtering, amplification, anddemodulation stages 180 and 182, to provide two respective DC voltagevalues, passed to the CPU via the analog-to-digital converter (ADC). Thecomparison of the two signals eliminates environmental effects (e.g.temperature, LED intensity), and is used to determine oxygenconcentration by the CPU.

[0109] Using the calculated flow volumes and oxygen concentrations, theperson's rate of consumption of oxygen is calculated by the CPU. Fromthis, the person's metabolic rate, in the form of Kcal/day, iscalculated and displayed on the liquid crystal display 18, using LCDcontrol circuitry 184.

[0110] The CPU also receives voltage signals from environmental sensors;temperature sensor 90, pressure sensor 92, and temperature sensor 94.These signals are also used in the calculations, as described below.

[0111] Calculation of Metabolic Parameters

[0112] As will be clear to those of skill in the art, there are a numberof ways to determine metabolic parameters such as VO₂ (volume of oxygenconsumed) and RMR (resting metabolic rate). As mentioned previously, thepresently preferred approach to determining metabolic parameters usesmeasurements of ambient temperature, pressure and humidity along withinhalation volume, exhalation volume, and oxygen concentration in theexhalation.

[0113] Initial Considerations

[0114] VO₂, the amount of oxygen consumed, is the difference between theamount of oxygen inhaled and the amount of oxygen exhaled. It is alsodesirable to determine VCO₂. VCO₂ is the volume of the carbon dioxideproduced by the body and is the difference between the amount of carbondioxide exhaled and the amount of carbon dioxide inhaled. RMR may becalculated once VO₂ and VCO₂ are known. Alternatively, certainassumptions may be made concerning the ratio between VO₂ and VCO₂,allowing RMR to be calculated from VO₂ alone. Therefore, a primarypurpose of the present invention is to determine VO₂. This requiresdetermination of both the amount of the oxygen inhaled and the amount ofoxygen exhaled. It is preferred to also determine VCO₂ as this allowsother metabolic parameters to be determined. To determine VCO₂ requiresmeasurement or calculation of both the amount of carbon dioxide inhaledand the amount of carbon dioxide exhaled. The method and calculationsused in the first preferred embodiment of the present invention arerepresented schematically in FIGS. 15 and 16.

[0115] Inhalation

[0116] The volume of oxygen inhaled, V_(i)O₂, may be calculated bymultiplying the volume of air inhaled by the fraction of that air whichis oxygen. The fraction of dry air that is oxygen varies only slightlyfrom location to location and can therefore be assumed to be 20.946percent. However, the actual air we breathe is not dry air, but insteadincludes a varying portion of water vapor. In order to determine theportion of the inhaled air which is oxygen, the volume of the inhalationwhich is attributable to water vapor must be determined and subtractedto provide a dry air measurement. As mentioned previously, a temperaturesensor 90, a relative humidity sensor 94, and an ambient pressure sensor92 are all mounted on the circuit board 88 inside the case of thereusable main portion 24 of the calorimeter 10. Theoretically, theseshould provide values for the temperature, pressure and humidity of theair inhaled by the user. However, under some conditions, the temperatureinside the case of the reusable main portion 24 may differ from ambienttemperature. This may be due to warming of the case by the user's hand,heating by the internal electronics, and heat absorbed from exhalations.Therefore, it is preferred that a correction be made to the temperaturevalues received from the ambient temperature sensor 90. If the relativehumidity sensor 94 were actually positioned in ambient air, instead ofinside the case, its output would reflect the relative humidity in theambient air. Since the relative humidity sensor 94 may be at an elevatedtemperature, its output indicates the relative humidity at this elevatedtemperature, rather than at true ambient conditions. Because the case isnot hermetically sealed, it is assumed that the partial pressure ofwater inside the case is the same as the partial pressure of water inthe surrounding atmospheric air. The partial pressure of water vapor,ppH₂O, can be computed from the following relationship:

ppH₂O=RH×VpH₂O(t)  (a)

[0117] where RH is relative humidity in percent, and VpH₂O is the vaporpressure of water, and t is temperature. VpH₂O is a function oftemperature and can be obtained from a look-up table or using anempirical curve fit. Therefore, the partial pressure of water vapor,ppH₂O, in the atmospheric air can be calculated from the known relativehumidity and temperature inside the case.

[0118] Referring to FIG. 16, a typical 750 mL total volume inhalation isshown at the far left side of the chart. This volume includes watervapor. The volume of water vapor in the inhalation may be determinedaccording to the following equation: $\begin{matrix}{{V\quad H_{2}O} = {\frac{{pp}\quad H_{2}O}{Pamb} \times V\quad {total}}} & (b)\end{matrix}$

[0119] where VH₂O is the volume of water vapor, ppH₂O is a partialpressure of water vapor, Pamb is the ambient pressure, and Vtotal is thetotal volume of the inhalation. In the example shown in FIG. 16, theambient temperature, pressure and humidity (ATP) are a temperature of23° C., a pressure of 755 mmHg, and a relative humidity of 35 percent.Using the above equations, the total volume of water vapor may be lookedup in a table or calculated to be 7.28 mL out of the 750 mL totalinspired volume. The amount of water vapor in the inhalation may then besubtracted from the total, giving a dry volume of 742.72 mL.

[0120] The percentage of dry air attributable to CO₂, O₂, and nitrogenand other gases is known from a variety of sources, examples of whichare given in the following chart: Component % of dry air Other (Argon)0.937 CO₂ 0.033 O₂ 20.946 N₂ 78.084

[0121] By multiplying these percentages by the total volume of dry air,the volume of each component gas may be calculated giving the valuesshown in the left-hand bar of FIG. 16. These volumes represent thevolumes of each of the component gases at ambient conditions.

[0122] As shown for the example of FIG. 16, the volume of oxygen inhaledat atmospheric conditions 155.57 mL. However, this is at atmosphericconditions, which vary from location to location and time to time.Therefore, it is necessary to convert the volumes of each of thecomponent gases to a standard temperature, pressure, and humidity, STPD(standard temperature and pressure, dry). The calculations typicallyused for RMR assume an STPD of 0° C., 760 mmHg and 0 percent relativehumidity.

[0123] As known to those of skill in the art, conversion between oneatmospheric condition and another is a simple matter of a ratio based ontemperature and pressure. However, in the present case, actualatmospheric temperature is not known because the temperature sensor maybe at an elevated temperature.

[0124] As known to those of skill in the art, the speed of sound is afunction of ambient temperature, the water vapor mole fraction, ambientpressure, and CO₂ mole fraction. This relationship is disclosed in TheJournal of the Acoustical Society of America, Vol. 93, No. 5, May 1993,pp. 2510-2516, the contents of which is incorporated herein byreference. The equation takes the form of: $\begin{matrix}\begin{matrix}{c = \quad {a_{0} + {a_{1}t} + {a_{2}t^{2}} + {\left( {a_{3} + {a_{4}t} + {a_{5}t^{2}}} \right)x_{w}} + {\left( {a_{6} + {a_{7}t} + {a_{8}t^{2}}} \right)p} +}} \\{\quad {{{\left( {a_{9} + {a_{10}t} + {a_{11}t^{2}}} \right)x_{c}} + {a_{12}x_{w}^{2}} + {a_{13}p^{2}} + {a_{14}x_{c}^{2}} + {a_{15}x_{w}{px}_{c}}},}}\end{matrix} & (c)\end{matrix}$

[0125] where the coefficients are defined in the following table:Coefficients Value a₀ 331.5024 a₁ 0.603055 a₂ −0.000528 a₃ 51.471935 a₄0.1495874 a₅ −0.000782 a₆ −1.82 × 10⁻⁷  a₇   3.73 × 10⁻⁸  a₈ −2.93 ×10⁻¹⁰ a₉ −85.20931 a₁₀ −0.228525 a₁₁   5.91 × 10⁻⁵  a₁₂ −2.835149 a₁₃−2.15 × 10⁻³  a₁₄ 29.179762 a₁₅ 0.000486

[0126] and where c is the speed of sound, t is the ambient temperature,x_(w) is the water vapor mole fraction, p is ambient pressure and x_(c)is the CO₂ mole fraction. Several of these variables have known values.The CO₂ mole fraction in ambient air may be assumed since its standardvalue is known and varies only slightly from location to location.Ambient pressure may be determined with high accuracy by the ambientpressure sensor in the calorimeter case. Also, the speed of sound may bemeasured by the flow meter during inhalation.

[0127] Because the ultrasonic flow meter preferably used with thepresent invention transmits ultrasonic pulses in both upstream anddownstream directions, the transit time, independent of flow speed, inambient air may be determined by averaging the upstream and downstreamtransit times during inhalation of ambient air. The speed of sound maythen be calculated according to the following equation.

c=L/2×(1/t _(u)+1/t _(d))  (d)

[0128] where c is the speed of sound, L is the distance between thetransducers, t_(u) is the transit time in the up direction, and t_(d) isthe time in the down direction.

[0129] This leaves essentially two variables, ambient temperature andwater vapor content. Relative humidity and ambient temperature areinterrelated by equation (a). Rearranging and solving for relativehumidity gives: $\begin{matrix}{{RH} = \frac{{pp}\quad H_{2}O}{{Vp}\quad H_{2}{O(t)}}} & (e)\end{matrix}$

[0130] At this point, the partial pressure of water, ppH2O is knownbased on the output of the humidity sensor 94 and the assumption thatthe partial pressure is the same inside and outside the case. However,equation (c) is expressed in terms of the water vapor mole fraction,x_(w), rather than relative humidity. Therefore, three additionalequations are required. The mole fraction of water vapor may becalculated as follows: $\begin{matrix}{X_{w} = {{RH} \times f \times \frac{p_{sv}}{p}}} & (f)\end{matrix}$

[0131] where RH is the relative humidity expressed as a fraction, f isthe enhancement factor, and p_(sv) is the saturation vapor pressure ofwater vapor in air:

f=1.00062+3.14×10⁻⁸ p+5.6×10⁻⁷ t ²  (g)

[0132] and $\begin{matrix}\begin{matrix}{p_{sv} = \quad {\exp\left( {{1.281\quad 180\quad 5 \times 10^{- 5}t^{2}} - {1.950\quad 987\quad 4 \times 10^{- 2}t} +} \right.}} \\{\quad {\left. {{34.049\quad 260\quad 34} - {6.353\quad 631\quad 1 \times {10^{- 3}/t}}} \right)P\quad {a.}}}\end{matrix} & (h)\end{matrix}$

[0133] When equations (c), (e), (f), (g) and (h) are combined, thisleaves two unknown variables, temperature and relative humidity. Asknown to one of skill in the art, the equations may be solved for thetwo remaining variables in a variety of ways. According to one presentlypreferred approach, equations may be solved through an iterativeprocess.

[0134] First, an initial temperature estimate is made. The temperatureindicated by the temperature sensor may be used as a starting point.Relative humidity is then determined according to equation (e). Then,the relative humidity just calculated is used in equation (c) tocalculate temperature. The calculated temperature is plugged back intoequation (e) to calculate a new relative humidity. The process isrepeated until the values converge, which typically occurs after severalrepetitions.

[0135] At the end of this process, the actual ambient temperature of theair being inhaled is known. Together with the measured ambient pressure,ambient conditions are now known. The volumes of each of the componentgases are then converted to STPD. Alternatively, or in addition to theabove approach, ambient temperature or temperature in the flow tube maybe directly measured using any type of suitable temperature sensor. Asone example, a temperature sensor may be mounted inside the case of thecalorimeter and a small fan could be used to continuously move ambientair past the sensor so that accurate readings are obtained. Otherapproaches will be clear to those of skill in the art.

[0136] When converted to STPD, the inhaled volume of gases have thevalue shown in the second bar of FIG. 16. As shown, the correctedinhaled volume of oxygen, V_(i)O₂ is 143.48 mL and the corrected inhaledvolume of carbon dioxide, V_(i)CO₂ is 0.23 mL.

[0137] Oxygen Sensor Calibration

[0138] As mentioned previously, the percent of oxygen in the inhaled airmay be measured or assumed. In the above explanation, the concentrationor percentage of oxygen is assumed, since this value varies onlyslightly from location to location. However, the oxygen sensor 84 doesrespond to the presence of oxygen in the inhalation. Therefore, theoutput of the oxygen sensor during inhalation may be used to calibratethe oxygen sensor as often as during each inhalation. In theory, anideal oxygen sensor varies its output only in response to changes in theconcentration of oxygen, and does not respond to changes in otherparameters such as temperature, humidity, and total pressure. However,the actual oxygen sensor is not entirely immune to changes in otherparameters.

[0139]FIG. 17 shows a series of curved surfaces representing the changein voltage output of the oxygen sensor with respect to changes in thepartial pressure of oxygen and an arbitrary second factor. This Figureis for illustration purposes only, and therefore the second factor maybe thought of as representing any or all of the other parameters towhich the sensor actually responds. Because the concentration of oxygenand the values for other parameters such as humidity, temperature, andpressure are known during inhalation, a point 190 may be plotted asrepresenting the combination of the known oxygen partial pressure andthe other factors. Extending upwardly from this point, it may be seenthat the theoretical or tested output curve 192 for the oxygen sensorpredicts an output voltage corresponding to point 194. If the actualvoltage output of the oxygen sensor under these known conditions differsfrom this value, a correction may be applied to the output curve tocorrect for this difference. For example, if the oxygen sensor actuallyputs out a voltage corresponding to point 196, a gain factor may beapplied to the primary output curve 192 so as to “move” the curve to thecurve shown at 198. This allows continual fine-tuning of the output ofthe oxygen sensor to improve its accuracy during measurement ofsubsequent exhalations.

[0140] Exhalation

[0141] During exhalation, total volume and oxygen partial pressure aremeasured using the flow meter and oxygen sensor respectively. As isknown to those of skill in the art, the temperature and humidity of anexhaled breath are reasonably constant from individual to individual.Specifically, the temperature of exhalation at the mouth averages 34.5°C. for most healthy individuals. Exhaled breath is also 100 percentsaturated with water vapor, giving 100 percent relative humidity.Experimentation with the present invention has established that thetemperature of exhaled breath averages approximately 32.5° C. at themidpoint of the flow tube 36. The pressure in the flow tube issubstantially identical to ambient pressure due to the low amount ofrestriction present in the calorimeter. The conditions of the exhaledbreath may be referred to as exhaled temperature pressure saturated(ETPS). In order to determine the volume of oxygen at ETPS, thefollowing equation is used.${V_{E}O_{2}} = {\frac{{pp}\quad O_{2}}{Pamb} \times {Vtotal}}$

[0142] where V_(E)O₂ is the exhaled volume of oxygen, ppO₂ is thepartial pressure of oxygen, Pamb is the ambient pressure, and Vtotal isthe total exhalation volume. In the example shown in FIG. 16, the totalexhalation volume is 800 mL, the partial pressure of oxygen is 121.6mmHg, and the ambient pressure is 755 mmHg. This gives an exhaled volumeof oxygen at ETPS of 128.05 mL. In order to make the RMR calculation, itis necessary to convert this value to STPD.

[0143] The exhaled volume of O₂ at ETPS may be converted to STPD byscaling for the differences in temperature and pressure. This gives anexhaled volume of O₂ at STPD of 114.43 mL. The volume of O₂ consumed bythe user during the single breath is calculated by subtracting theexpired volume of oxygen from the inspired volume of oxygen. Multiplyingby the number of breaths during a minute gives the amount of oxygenconsumed during a minute.

[0144] Preferably, the production of CO₂ should also be determined. Inorder to do this, additional calculations are required. First, certainassumptions may be made about the temperature and humidity of exhaledbreath. The volume of water vapor in the exhaled breath may bedetermined from the assumed relative humidity and temperature, and themeasured flow volume. Removing water vapor to convert to dry air leavesa total volume of 761.68 mL. Also, it is assumed that nitrogen (N₂) andtrace gases are conserved in the lungs. Therefore, the volume ofnitrogen and trace gases inhaled equals the volume of nitrogen and othergases exhaled at STPD. This assumption improves as data is summed formultiple breaths.

[0145] As shown in FIG. 16, the volume of nitrogen and trace gases maybe converted from STPD to ETPS, giving a volume of 605.74 mL. At thispoint, the volume of water vapor, oxygen, and nitrogen and trace gasesis known at ETPS. Also, the total volume is known. Therefore, the volumenot accounted for by water vapor, oxygen, and nitrogen and trace gasesis attributable to CO₂. This gives a CO₂ volume of 27.89 mL at ETPS.This value is then converted to STPD, giving an exhaled volume of carbondioxide of 24.92 mL at STPD. The volume of CO₂ produced by the userduring the single breath is calculated by subtracting the inspiredvolume of carbon dioxide from the expired volume of carbon dioxide.Multiplying by the number of breaths during a minute gives the amount ofcarbon dioxide produced during a minute.

[0146] Calculation of Resting Metabolic Rate

[0147] As known to those of skill in the art, resting metabolic rate(RMR) may be calculated in a variety of ways. One known and acceptedapproach is given by the de Weir formula, which takes the form:

RMR=1.44(3.581×VO₂+1.448×VCO₂)−17.73

[0148] where VO₂ is the volume of oxygen consumed inmilliliters-per-minute, VCO₂ is the amount of CO₂ produced inmilliliters-per-minute, and RMR is the resting metabolic rate in Kcalper day. As an alternative, certain assumptions may be made concerningthe ratio between VO₂ and VCO₂. Specifically, the respiratory quotientis given by the following formula:${RQ} = \frac{V\quad {CO}_{2}}{V\quad O_{2}}$

[0149] where RQ represents respiratory quotient. The respiratoryquotient typically ranges between 0.7 and 1.1 depending on the type ofstored energy source being metabolized by the user's body. RQ may beassumed to be 0.85 for typical users during the calculation of restingmetabolic rate. Therefore, using this ratio and substituting for VCO₂gives the equation:

RMR=6.929×VO₂−17.73

[0150] where RMR is resting metabolic rate in Kcal per day, and VO₂ isthe volume of oxygen consumed by the user in milliliters-per-minute.Preferably, the various parameters which are measured by the calorimeterare summed or averaged over multiple breaths, thereby giving improvedaccuracy.

[0151] As an alternative, a CO₂ sensor may be incorporated into thecalorimeter so as to directly measure, rather than calculate, CO₂concentrations. This allows more accurate calculations of RMR as well ascalculation of RQ.

[0152] Use of the Calorimeter

[0153] When the calorimeter is first turned on, the unit goes through awarm up and calibration period. During this time, the oxygen sensorheater is turned on and warms the oxygen sensor to a steady state value.During this time, the oxygen sensor is also turned on. Once the oxygensensor has reached steady state, a zero-flow test is performed. Duringthe zero-flow test, the flow sensor measures flow speed through the flowtube. Since the calorimeter is not being used at this stage, thereshould be zero flow through the flow meter. However, if the flow meterindicates a slight flow in one direction or another, an offset isassigned to reestablish zero. A variety of approaches to this zeroingmay be used, though it is preferred that multiple readings are takenprior to application of an offset factor. Also, during an actual test,the flow meters may be dynamically re-zeroed during known periods ofzero flow.

[0154] To use the calorimeter to calculate a subject's resting metabolicrate (RMR), it is preferred that the subject sit or relax in acomfortable position and then bring the respiratory connector intocontact with their face or mouth, after the calorimeter has been turnedon and allowed to warm up and self-calibrate, as previously described.The subject then breathes normally through the calorimeter for a periodof several minutes. Typically, users require some amount of time beforetheir breathing and measured metabolic rate stabilizes. Therefore, it ispreferred that initial data not be used as an indication of restingmetabolic rate. As will be clear to those of skill in the art, there area variety of approaches which allow the calorimeter to most accuratelydetermine resting metabolic rate. According to one preferred approach,once the calorimeter detects breath flow through the calorimeter, itwaits 30 seconds then begins recording. However, this period of time maybe increased or decreased. Once recording begins, the calorimeter makesmeasurements of flow, oxygen concentration, and speed of sound. Oxygenpartial pressure is measured every tenth of a second, and flow velocityand speed of sound are measured 200 times per second. Flow velocity andspeed of sound measurements are averaged so as to obtain a value everytenth of a second for computation of volumes. The calorimeteraccumulates this data to calculate volume inspired, volume expired,inspired oxygen concentration (for calibration purposes), expired oxygenconcentration, ambient temperature, ambient humidity, and ambientpressure. Ten breaths are then averaged in order to obtain one breathblock. At the end of each breath block, VO₂ is calculated for the block.In order to determine steady state, three blocks are checked to seewhether they are within a certain percentage of each other. For example,if the previous two blocks are both within 7 percent of the currentblock, the block is flagged as steady state. It is determined thatsteady state has been reached when a certain number of consecutiveblocks are flagged as steady state, such as four or five breath blocks,and then VO₂ and VCO₂ are used to calculate RMR, which is displayed onthe display 18. Typically, people take 8 to 10 breaths per minute so abreath block is about one minute long. Obviously, the data may beprocessed in other ways. Also, certain error states may be indicated.For example, if breathing is occurring too rapidly or too slowly, anerror signal may be indicated. Also, errors may be indicated for toohigh of a flow rate, an RMR that is out of an acceptable range, forhardware errors, or for other reasons.

[0155] As mentioned previously, it takes most users some time tostabilize their breathing and indicated rested metabolic rate. However,according to another aspect of the present invention, data during the“settling down period” may be used to predict the data during the steadystate period.

[0156] A person should be fully relaxed for the measured metabolic rateto be the rest metabolic rate. However, the person's breathing willoften be affected by the presence of the mouthpiece or mask,particularly during the time immediately following placing the mask overthe person's nose and mouth. Accurate measurements may be delayed acertain time period, e.g. 2 minutes, after the mouthpiece has been putin place, after which the person's breathing may return to normal.However, the person may not feel comfortable with the mouthpiece inplace for so long.

[0157] In order to reduce the time necessary to determine an accuratevalue of metabolic rate of a person, algorithms may be used to extract aresting level of VO₂ from data that is tending towards the restingvalue. FIG. 18 illustrates a possible data set of VO₂ measurements (andhence measured metabolic rate) vs. time for a person obtained using anindirect calorimeter. Oxygen consumption is measured as a function oftime, e.g. breath by breath, or by blocks of a certain number ofbreaths, e.g. 10. In FIG. 18, the measured oxygen consumption, shown bya solid line, approaches a value corresponding to the true restingmetabolic rate, shown by a dashed line, as time advances. The person'sactual metabolic rate may be constant during the measurements, with VO₂measurements initially high due to breathing anomalies, but in othercases the metabolic rate itself may fall slowly towards a true value ofrest metabolic rate. Both cases can be modeled. The obtained data is fitto a mathematical equation, (e.g. in terms of polynomials, exponentials,logarithmic functions, other functions, etc.) in terms of a number ofparameters, including the resting metabolic rate. The resting metabolicrate is determined from a fit to the data, and the error in thismeasurement is estimated from the quality of the fit to the data. Thisprocess can be executed continuously in real time, as the respiratoryanalysis proceeds, so that the measurements can be stopped, and themouthpiece removed, once an accurate measurement has been made.Alternatively, the data can be saved and the numerical analysis madeafter the test is complete.

[0158] The exact form of the data fit used will depend on the person'sresponse to the mouthpiece and other testing conditions. In thisexample, for the case illustrated in FIG. 18, data might be fitted to anexpression of the form

VO₂ =A+B exp (−t/C)

[0159] where A is the value of VO₂ corresponding to the true restingmetabolic rate, B is a measure of breathing abnormality at the onset oftesting, and C is a measure of how quickly breathing returns to normalafter the beginning of the test. After a number of initial tests on aperson, a suitable equation can be chosen to model that person'sbreathing response to testing. Alternatively, a model may be chosenbased on the age or other demographic data relating to the person. Thefirst breath, or first few breaths, may be discarded from the data toimprove the fitting.

[0160] Subsequent respiratory analysis may then be shortened by thisanalysis, e.g. using a method described below, or other method.

[0161] (a) After initial testing, the time taken for breathing to fallto close to normal can be determined, and hence used to determine thelength of the testing. Data can be excluded from the first part of thetest, and averaged over the remaining measurements. For example, if theabove equation is applicable, data obtained before some multiple(integer or fractional) of C has passed may be discarded (e.g. if C=10seconds, data taken during the first 30 seconds of the test may bediscarded, and the remaining data averaged).

[0162] (b) If the data from the test is being analyzed in real time, thetest can be ended once an acceptable fit to the data has been obtained.

[0163] (c) Data may be viewed by a professional as the test is inprogress, and the test stopped once the professional judges data ofsufficient quality has been determined. This judgment will be based onexperience.

[0164] For a person breathing through the calorimeter of the presentinvention, the data can be stored by the calorimeter time, and thentransmitted to another electronic device for display, analysis, etc.Data may also be transmitted to another electronic device while the testis in progress (i.e. in ‘real time’). Data transfer from the calorimeterto another device may use flash cards (memory cards), wirelesstransmission (e.g. Bluetooth), cables, IR transmission, or otherelectromagnetic or electrical methods, or by plugging the calorimeterinto the other device. The use of flash cards is disclosed more fully inMault's provisional patent application serial No. 60/177,009 filed Jan.19, 2000, and incorporated herein by reference. The calorimeter mayfurther comprise computing means for performing data analysis.

[0165] Under certain circumstances, a user may never reach steady stateduring a test. Under these circumstances, the calorimeter may indicatethat no reading was possible, or a steady state value may be estimated.According to one approach, the breath blocks during the test may beaveraged with some additional weighting given to blocks towards the endof the test when it is assumed that the user is closer to steady state.Obviously, detailed data recorded by the calorimeter may be observed byan experienced professional to determine the reliability of the data.For example, the calorimeter may be interconnected with a desktopcomputer which records and/or displays data on ameasurement-by-measurement or breath-by-breath basis. In this way, theprofessional may observe that the subject is having trouble reachingsteady state and may provide counseling or suggestions on how to betterinteract with the device. Also, the detailed data may provide othervaluable indications about the subject.

[0166] Calorimeter Embodiments with Improved Hygiene

[0167] It is preferred that a calorimeter according to the presentinvention be able to safely be used by multiple users without undue riskof transferring pathogens from one user to another. In the previouslydiscussed preferred embodiment of the present invention, each individualuser is given their own disposable portion along with its respiratoryconnector. A fitness facility or a doctor may then own the reusableportion. As an alternative, each individual user may own a completecalorimeter and the disposable may merely be removable for cleaningpurposes. However, it is preferred that the calorimeter be designed suchthat pathogens are not easily transferred from one user to another.Several improved sanitation versions of the present invention aredisclosed in FIGS. 19-26 and an alternative oxygen sensor configurationis shown in FIG. 9.

[0168] Referring first to FIG. 19, a calorimeter according to thepresent invention is generally shown at 210. This calorimeter has areusable main portion 212 that is similar to the reusable main portion24 discussed earlier. However, in the embodiment shown in FIGS. 3 and 4,the user's inhalation and exhalations may come in contact with theultrasonic transducers 80 and 82, the oxygen sensor 84, and the surfacesin the outlet flow passage 60. These form part of the reusable portionand therefore are not disposed or changed from user to user. Theembodiment of FIG. 19 is altered so as to prevent contact of the user'sbreath with the transducers and oxygen sensor. The disposable portion214 has a ceiling 216 closing off the upper end of outer shell 218 and afloor 220 closing off the lower end of the outer shell 218. A hole 222in the ceiling 216 aligns with the upper ultrasonic transducer 224 andhas a piece of germ barrier material 226 disposed in the hole 222. Thebarrier material may be any of a variety of materials that block thepassage of pathogens but allows a passage of ultrasonic pulses.Likewise, a hole 228 is defined in the floor 220 that aligns with thelower ultrasonic transducer 230. A piece of germ barrier material 232 isalso disposed in this hole 228. The oxygen sensor 234 in this embodimentis moved upwardly somewhat compared to the earlier disclosed embodiment.An opening 238 is formed in the back wall 236 of the recess in the mainportion 212 with the opening 238 aligning with the oxygen sensor'sforward sensing surface. The outer shell 218 of the disposable 214 has arearward wall 240 extends down past this opening 238 and joins with thefloor 220 of the disposable portion 214. An opening 242 is defined inthis rearward wall 240 and a membrane 244 is disposed across theopening. The membrane is of the type that allows free passage of oxygento the oxygen sensor, but does not allow passage of pathogens. A passage246 is cut in the floor 220 of the disposable portion 214 allowing flowto pass into an outlet passage 248 defined in the reusable portion. Thispassageway 248 is large and has smooth sides to allow easy flow ofinhalations and exhalations. The side walls of this passage 248 may becoated with an anti-bacterial and/or anti-viral substance to preventcontamination. Alternatively, the passageway may be cleaned betweenuses. As a further alternative, a disposable sleeve may be inserted intothis passageway, which mates with the opening in the floor of thedisposable portion. The sleeve would also be removed and disposedbetween users.

[0169] Referring now to FIG. 20, another alternative improved sanitationversion of a calorimeter according to the present invention is generallyshown at 250. As with the previously described version, the disposableportion 252 of the calorimeter 250 includes a ceiling 254 closing offthe upper end of the outer shell 256 and a floor 258 closing off most ofthe lower end. In this version, a thin micromachined ultrasonictransducer 260 is mounted to the lower side of the ceiling 254 of thedisposable portion 252 directly above the upper end of the flow tube262, which forms part of the disposable portion. This thin ultrasonictransducer 260 replaces the larger ultrasonic transducers discussed inthe earlier embodiments. The transducer may be a micromachinedultrasonic transducer array such as the ones produced by Sensant of SanJose, Calif.

[0170] Electrical contacts 264 are disposed in the rear wall 266 of thedisposable portion 252, directly behind the transducer 260 and areelectrically connected, such as by wires 268, to the transducer 260.Corresponding electrical contacts 270 are disposed on the rear wall 272of the recess in the reusable portion 274 of the calorimeter 250 andalign with the contacts 264 on the disposable portion 252. The contacts270 on the reusable portion are in turn wired to the main circuit board276. Therefore, once the disposable portion 252 is docked in thereusable portion of the calorimeter, the thin ultrasonic transducer 260is in electrical communication with the main circuit board 276. However,because the thin transducer 260 and its associated wiring are mounted inthe disposable portion 252, the entire transducer may be disposed alongwith a remainder of the disposable portion. This prevents any concernsabout contact of the user's breath with the transducer. Alternatively,the disposable portion may be designed so as to be cleaned according toa specified cleaning procedure that does not harm the transducers.

[0171] A lower thin ultrasonic transducer 278 is disposed on the uppersurface of the floor 258 of the disposable portion 252, aligned with aflow tube 262, and cooperates with the upper transducer 260 to measureflow through the flow tube. Like the upper transducer 260, the lowertransducer 278 is wired to electrical contacts 280 that abut electricalcontacts 282 disposed on the rear wall 272 of the recess. A passage 284is defined in the floor 258 of the disposable portion 252 so as to allowinhalation and exhalation to flow in and out of the disposable portion.This passage communicates with a large flow area 286 in the bottom ofthe reusable portion 274 of the calorimeter. As an alternative, theentire lower portion of the reusable portion may be removed so that thepassage in the floor of the disposable portion has no part of thereusable portion directly below it. In this way, inhalation andexhalation flowing through the passageway flows directly to and from thesurrounding ambient air without coming into contact with any part of thereusable portion.

[0172] This embodiment of the calorimeter also uses an alternativeversion of an oxygen sensor 288. In this version, the LED and photodiodeportions of the oxygen sensor are incorporated in a sensor package 290disposed in the rear wall 272 of the recess approximately midway betweenthe upper and lower ends of the recess. The remainder of the oxygensensor 288 forms a part of the disposable portion 252 and is referred toas the fluorescence portion 292. The fluorescence portion 292 consistsof a light pipe 294 extending from the rear surface 296 of the outershell 256 adjacent the sensor package 290 into the wall 298 of the flowtube 262. The fluorescence material 300 is disposed on the end of thelight pipe 294 so that it is in contact with the gases flowing throughthe flow tube 262. The light pipe 294 conducts light traveling to andfrom the fluorescence material 300. This configuration allows disposalof the portion of the oxygen sensor 288 that comes into contact with theuser's breath. As shown, the fluorescence material 300 is positionedapproximately midway in the flow tube 262. This provides a benefit inthat the portion of the flow that is being sensed by the oxygen sensoris approximately at the midpoint of the portion of the flow that isbeing measured for flow speed. This allows better time correlation ofthe flow and oxygen concentration measurements.

[0173] Referring now to FIG. 9, yet another alternative version of anoxygen sensor 302 is disclosed. In this version, the sensor package 304is interconnected with a circuit board 306. The sensor package 304includes the light emitting diode, LED, and photodiode of the earlierdiscussed embodiment. Pieces of fluorescence material 308 are disposedin the wall 310 of a flow tube 312, a portion of which is shown. Lighttravels between the sensor package 304 and the fluorescence material 308across a small air gap. Obviously, this configuration requires adifferent construction of the flow tube. However, it allows simple andcompact construction of an oxygen sensor with a disposable portion.

[0174] An important factor in the disclosed oxygen sensors withdisposable portions is calibration. A fluorescence quench oxygen sensorof the type described herein typically requires careful calibration forthe chemistry used. However, highly accurate and repeatable applicationof fluorescence material reduces the need for individualizedcalibration. Instead, the sensor package may include a mathematicalmodel of the fluorescence material such that accurate oxygenconcentration measurements may be made with disposable fluorescencematerials. As discussed previously, calibration of the oxygen sensorduring inhalations further improves accuracy.

[0175] Referring now to FIG. 21, an alternative approach to improvedsanitation for use with a calorimeter according to the present inventionis illustrated. A calorimeter body according to any of the embodimentsof the present invention is generally shown at 320. A germicidalfiltration module 322 connects between the inlet conduit 324 of thecalorimeter 320 and the respiratory connector, here shown as amouthpiece 326. Referring to both FIGS. 21 and 22, the module 322 has afilter housing 328 with a calorimeter port 330 defined on one side and arespiration port 332 defined in the other. The calorimeter port 330mates with the inlet conduit 324 of the calorimeter while therespiration port 332 mates with the respiration connector. The housing328 may be of various shapes, including the generally rectangularconfiguration shown in FIG. 21. A piece of biological filter material334, such as Filtrete® from 3M, extends within the housing 328 such thatair flowing between the respiration port 332 and the calorimeter port330 must pass through the filter material. The filter material isoperable to remove pathogens thereby preventing pathogens from flowingfrom the respiration connector into the calorimeter. In this way, thecalorimeter remains sanitary during use. Each subsequent user uses a newfilter module 322 with the used module either being retained by thatuser or disposed.

[0176] Referring again to FIG. 22, it can be seen that the module 322has two generally parallel and spaced apart side walls 336 with aperimeter edge 338 interconnecting the side walls 336. The filtermaterial is generally parallel to the side walls 336 and extends betweenthe perimeter edges 338. As best shown in FIG. 22, a saliva retentionwall 340 extends upwardly from the bottom edge adjacent the filtermaterial 334 on the side of the filter material closest to therespiration connector 326. During use of the calorimeter, especiallywith a mouthpiece, saliva is entrained in the exhalation breath and ispreferably not introduced into the calorimeter. Much of the entrainedsaliva will flow along the lower edge of the respiration port 332 anddown the inside of the side wall 336 where it will collect in the areabetween the saliva retaining wall 340 and the side wall 336, as shown.Also, some entrained saliva may contact the filter material and thenfall downwardly to collect in the saliva trap. This arrangement avoidsthe need for the saliva trap discussed earlier in the disposable portionof the calorimeter, though it may be retained for other purposes.

[0177] Referring now to FIGS. 23 and 24, an alternative hygiene barrierarrangement is illustrated. In the configurations of FIGS. 23 and 24, amask 342 is provided instead of a mouthpiece. In this case, the mask 342consists of a semi-rigid outer shell 344 that interconnects with theinlet conduit 346 of the calorimeter 348. The mask shell 344 may be madeof any of a variety of materials, including polystyrene. The mask shell344 is preferably ultrasonically bonded to the inlet conduit 346 of thedisposable portion of the calorimeter to provide an air-tight seal. Adisposable mask liner 350 is inserted into the mask shell 344. The maskliner 350 includes a liner shell 352 which overlies a portion of themasked shell 344, a face seal 354 to seal the mask 342 to the face ofthe user, and a hygiene barrier 356 that filters all gases flowing intoand out of the calorimeter. Once again, the hygiene barrier 356 may be amaterial such as Filtrete® by 3M. The face seal 354 preferably is aninflated sealed film that easily forms to the shape of the user's faceproviding a secure seal. The face seal 354 is securely attached, such asby a cement bond, to the liner shell 352, which is preferably a vacuumformed plastic. The hygiene barrier 356 is securely interconnected withthe liner shell 352 such as by an ultrasonic bond.

[0178] Referring now to FIGS. 25 and 26, an alternative filtered maskdesign 360 is disclosed. Similar to the previous version, a semi-rigidmask shell 362 is interconnected with the inlet conduit 364 of thedisposable portion 366 of the calorimeter 368. A mask liner 370 insertsinto the shell and is disposable. The mask liner 370 includes a piece ofhygiene barrier material 372 such as Filtrete® which is interconnected,such as by insert molding, to a liner shell 374 which is in turn moldedwith an injection molded-type face seal 376 of elastomer material. Theface seal 376 securely seals to the face of the user thereby preventingleakage.

[0179] Because users vary in the size and shape of their face, maskshells and/or mask liners may be provided in a variety of sizes andshapes to suit various users. Also, as will be clear to those of skillin the art, other designs of masks and filter housings may also be usedwherein the breath is filtered. According to the present invention, itis preferred that a relatively large piece of hygiene barrier materialis used so as to prevent a pressure drop across the material. In thisway, the barrier material does not significantly increase the resistanceof flow through the calorimeter and thereby does not cause theexpenditure of additional energy during use of the calorimeter.

[0180] As an alternative, a mask according to the present invention mayinclude a nares spreader for opening the nostrils of a user, therebyreducing the effort associated with breathing through the mask. As oneapproach, adhesive pads may be provided inside the nose portion of themask. The pads are pressed into contact with the nose of the user and,when released, the mask opens the nasal passages.

[0181] Other Alternative Designs

[0182] The above discussed embodiments of the present invention may bealtered in various ways without departing from the scope or teaching ofthe present invention. The following are a number of alternative designsand alterations on the preferred embodiments.

[0183] While the preferred embodiments of the present invention utilizea fluorescence based oxygen sensor, other approaches may also be used.Other possible oxygen sensor methods include solid oxide sensors ifadapted for rapid response, e.g. using zirconium oxide; or otherelectrochemical sensors. Molecular fluorescence, e.g. laser-inducedfluorescence, may also be used. For example, laser radiation can be sentalong the flow path, and fluorescence detected using a sensor on theside of the flow path, or light guides used to convey fluorescence to adetector in the reusable body of the device. Similarly Ramanspectroscopy, including nonlinear Raman spectroscopy, may be used. Alaser beam might pass along the flow path, with detection in a directionat some angle to the beam. Narrow-band filters, to remove laserradiation, would aid in detection, as would phase-sensitive detection.Other oxygen-detection techniques include laser absorption;chromatography methods; sensors based on diffusion rates through films;or fast-response colorimetric sensors, e.g. using the photoabsorption orphotoreflectance changes of films, such as transition metal complexes,in the presence of oxygen. IR emission from vibrationally excitedmolecules may be detected. Laser radiation might be used for selectivevibrational or vibronic excitation of molecules. Also, phosphorescentcompounds, e.g. platinum and gold complexes, are useful for oxygendetection, as described in e.g. A. Mills, Platinum Metals Review, June1997; U.S. Pat. No. 5,119,463; and elsewhere. Selective (e.g. laser)photoionization of molecules, followed by detection of photoions andelectrons, may provide a photocurrent proportional to molecularconcentration. The ultrasonic spectrum of the respired gas may alsocontain molecular information, related to concentration, particularly ifwide-spectrum response (up to 10 MHz and higher frequencies)micromachined ultrasonic transducers are used.

[0184] As mentioned previously, the preferred oxygen sensing capabilityof the present invention may be supplemented by the addition of a carbondioxide sensor. Other gases may be sensed as well. Generically, oxygensensors, carbon dioxide sensors, as well as other gas sensors arereferred to herein as component gas concentration sensors. Carbondioxide sensing may be accomplished in a variety of ways. Carbon dioxideconcentration may be measured using a carbon dioxide scrubber incombination with volume measurements as described in some of Mault'searlier patents and applications. Also as described in some of Mault'searlier patents and applications, metabolic calculations may be madebased on measurement of carbon dioxide, without the measurement ofoxygen. A calorimeter according to the present invention may beconstructed with any of a variety of carbon dioxide sensors, such as acapnometer, and without an oxygen sensor. Carbon dioxide may be measuredusing IR absorption, using the strong carbonyl absorption, or otheranalytical techniques, such as those listed earlier for oxygen. Carbondioxide and oxygen sensors may be combined into the same package for acombined fluorescent quenching sensor, for example, using selectivelypermeable membranes or different fluorescent compounds.

[0185] As other approaches to indirect calorimetry, the approachesdisclosed in Mault's PCT WO 00/07498, incorporated herein by reference,may be incorporated into a calorimeter constructed according to thepresent invention. Specifically, the oxygen sensor could be omitted andthe mass flow determined based on either approach in WO 00/07498. Thisavoids the cost associated with the oxygen sensor. Alternatively, themass flow based approach may be used as a supplement to one or more gasconcentration sensors.

[0186] As yet another approach to indirect calorimetry, a carbon dioxidescrubber may be used to remove substantially all of the carbon dioxidefrom the inhalation and/or exhalation flow, and the difference in flowvolume measured to determine the amount of carbon dioxide produced. Fromthis, metabolic rate may be determined. This avoids the need forcomponent gas concentration sensors. Instead, only a scrubber and a twoway flow meter are required. This approach is further disclosed inMault's U.S. Pat. No. 5,179,958. The above described embodiments of thepresent invention may easily be configured to utilize this approach. Forexample, a scrubber module may be inserted in the flow path between thedisposable portion and the respiratory connector, as part of or in placeof the hygiene filter module of FIG. 21. Alternatively, the disposableportion may be designed to include scrubber material in an extended flowpath.

[0187] Other flow sensing methods are possible, for example, using thecooling rate or heat dissipation of objects in the flow path. Hot wiremass sensors are known in the art, along with analogous devices usingsemiconductors (e.g. silicon), ceramics, etc, e.g. hot filmsemiconductor sensors. Other methods include turbines or impellers;noise levels as gas flows e.g. around an obstruction or through anaperture; distortion of e.g. an aperture or membrane due to the pressuredifference between each side, which could be monitored with highprecision using e.g. laser reflection; or distortion of other structuresplaced in the flow path, e.g. micromachined rods; and thermoelectric gasflow sensors. Direct pressure difference measurements may be used e.g.using micromachined pressure sensors at either end of a flow path. Otherconfigurations of ultrasonic transducers are also possible. For example,three transducers could be mounted at the edges of a gas flow path,forming a V-shaped configuration. The transducer at the center of the Vwould transmit to two other transducers mounted on the opposite side ofthe flow path, spaced an equal direction on either side of the centertransducer. The difference between the two transmission times is relatedto gas flow velocity. Other flow measurement techniques include thermalimaging of the flow path, followed by image analysis; the Doppler shiftof transmitted ultrasonic signals; or Doppler shift or broadening ofmolecular or atomic absorption or emission bands, as measured using e.g.laser radiation.

[0188] The problems related to moisture may be reduced by protecting theoxygen sensor from moisture, or removing the moisture from the air flow.For example, moisture removal may include passing the exhaled gasthrough or past, foam sheets (possibly fabricated to include a dryingmechanism); zeolites; molecular sieves; membranes; chemical dryingagents, e.g. silica gel. These moisture-removing means could be mountedwithin a removable part, for easy replacement. The oxygen sensor may beprotected from the effects of moisture using e.g. a water-impermeable,oxygen permeable membrane placed over the oxygen sensor, or hydrophobicfilms placed over the sensor

[0189] Other methods for measuring the temperature of the gas flowinclude detecting thermal distortion of micromachined structures in theflow path, e.g. of multilayer membranes using optical or electricalmethods; or by monitoring temperature-dependent molecular or atomicproperties, e.g. emission or absorption wavelengths. Computer modelingof respired air temperature as it passes through the device may becombined with spot temperature measurements to obtain a detailedtemperature distribution. Thermoelectric sensors, thermistors,pyroelectric sensors, thermopiles, etc. may be used. The temperaturedependence of the ultrasonic spectrum of inhaled air may be monitored.Thermal imaging of the flow path may also be useful.

[0190] In addition to the present embodiment, there are many otheradaptations of the present invention (sometimes referred to as “thedevice” below) which may be useful. For example, the air vents of thedevice may be replaced with a connector adapted to send exhaled air toother analytical devices for further analysis. Other gas sensors may beincluded in the flow path. Respiration components of interest include:oxygen and carbon dioxide (as previously discussed), nitric oxide, otherradicals, ketones (e.g. acetone), aldehydes (e.g. acetaldehyde), alkanes(e.g. pentane), other hydrocarbons, esters, hydrogen sulfide, indicatorsof lung disease or cancer, other volatile organic compounds, gasesproduced by bacteria (e.g. sulfides). Detectors for radioisotopes ofinert gases (e.g. xenon) may be included for quantitative lung functiontests.

[0191] The embodiments of the present invention thus far describedassume inhalation of atmospheric gases. However, the present inventionis equally applicable to inhalation of other gas mixtures from a sourceof respiratory gases. For example, a connector may be provided on thebottom the calorimeter, in addition to or in place of the vents, so thatthe calorimeter may be interconnected to a source and/or sink orrespiratory gases other than atmospheric. One application of such anapproach is the use of a calorimeter according to the present inventionin an anesthesiology or assisted breathing apparatus. The flow throughthe calorimeter may be assisted in either direction and pressures otherthan atmospheric may be utilized. Obviously, sensors would be used tomonitor these non-atmospheric conditions so that the proper calculationsof metabolic rate and other respiratory factors may be made. Additionalaspects concerning the use of a calorimeter according to the presentinvention as part of a mechanical ventilation system will be clear froma review of Mault's provisional patent application Nos. 60/179,906 filedFeb. 2, 2000 and 60/179,961 filed Feb. 3, 2000, both of which areincorporated herein by reference.

[0192] Breath profile analysis may be used e.g. in order to determineend tidal volumes precisely, or investigate breathing anomalies due toe.g. blockages. The device may communicate with other physiologicalsensors, and/or be in communication with other electronic devices, e.g.for data transmission, data analysis, display, feedback, or other uses.Data from spirometry/indirect calorimetry obtained using the presentinvention may be combined with other physiological or environmental datafor analysis. The device may produce electromagnetic radiation forpowering physiological sensors embedded in the body of the person undertest, e.g. micromachined ultrasonic flow sensors placed near the lungs,arteries, or veins. Also, a calorimeter according to the presentinvention may include other sensors or physiological monitors. Forexample, a positioning system, based on GPS, telemetry, cellular phonesignals, or others may be incorporated to provide information on thelocation of a user. The calorimeter could then be used during anexercise session that requires moving around, and the positioning systemwould provide information on position while the calorimeter providesmetabolic information, allowing correlations and analysis.

[0193] While the present invention is preferably directed to themeasuring respiratory parameters such as metabolic rate, a simpler flowmeter version of the present invention is also of merit. The presentinvention, with the oxygen sensor removed, and possibly simplified inother ways, provides an excellent flow meter for such applications asmeasuring flow rate and volume in lung capacity tests. The flow metercould also be used in other applications.

[0194] A calorimeter according to the present invention may beincorporated into a weight or health management system, which mayinclude a personal digital assistant (PDA) for data entry,communication, physiological monitoring, feedback, and data processing.This and other uses for the present invention are disclosed in Mault'sprovisional applications serial No. 60/165,988 filed Nov. 17, 1999; No.60/167,276 filed Nov. 24, 1999; No. 60/177,016 filed Jan. 19, 2000.

[0195] Other physical configurations of the present invention arepossible without departing from the scope or teaching. For example, thedisplay for displaying metabolic parameters may be repositioned,reconfigured, or supplemented. The display could be moved to a positionsuch that the subject could see the display during a test.Alternatively, a separate display, which received data either through awire or wirelessly from the calorimeter, may be provided so that a usermay position the display where it is easy to read during a test. Thedisplay could also or alternatively be viewed by another person such asa health professional. Viewing the display during testing could allowthe user to witness metabolic changes due to changes in their activitylevel, relaxation level, or for other reasons. For example, thecalorimeter and the display, or other feedback device, could be used abiofeedback system for helping people to reach certain levels ofrelaxation. Breathing therapy and training could also be administeredusing the calorimeter to monitor breathing rate, volume, and otherfactors.

[0196] As yet another alternative, an artificial “nose” may be providedfor use with or as part of the calorimeter. An artificial “nose”conditions the inhalations and/or exhalations so as to control humidityor temperature. This may be advantageous for some applications.

1. An indirect calorimeter for measuring the metabolic rate of asubject, said calorimeter comprising: a disposable portion having arespiratory connector configured to be supported in contact with thesubject so as to pass inhaled and exhaled gases as the subject breathes;a flow pathway within said disposable portion operable to receive andpass inhaled and exhaled gases, said flow pathway having a first end influid communication with said respiratory connector and a second end influid communication with a source and sink for respiratory gases; areusable portion having a housing with a recess defined therein forreceiving said disposable portion; a flow meter within said housingconfigured to generate electrical signals as a function of theinstantaneous flow volume of inhaled and exhaled gases passing throughsaid flow pathway; a temperature sensing means within said housingoperable to generate an electrical signal representative of ambienttemperature; an ambient pressure sensing means within said housingoperable to generate an electrical signal representative of ambientpressure; a humidity sensing means within said housing operable togenerate an electrical signal representative of relative humidity; acomponent gas concentration sensor within said housing operable togenerate an electrical signal as a function of the instantaneousfraction of a predetermined component gas in the exhaled gases as thegases pass through said flow pathway; and a computation unit operablewithin said housing to receive said electrical signals from said flowmeter, said temperature sensor, said humidity sensor, said ambientpressure sensor and said concentration sensor and operative to calculateat least one respiratory parameter for the subject as the subjectbreathes through the calorimeter.
 2. The calorimeter according to claim1, wherein said flow pathway includes an elongated flow tube throughwhich the inhaled and exhaled gases flow, and a chamber disposed betweensaid flow tube and said first end, said chamber being a concentricchamber surrounding one end of said flow tube and being defined betweensaid flow tube and said outer housing.
 3. The calorimeter according toclaim 1, wherein said disposable portion is received in the recess in adirection perpendicular to said flow tube.
 4. The calorimeter accordingto claim 1, wherein said component gas concentration sensor is an oxygensensor.
 5. The calorimeter according to claim 4, wherein said oxygensensor is a fluorescence quench type oxygen sensor.
 6. The calorimeteras set forth in claim 4 wherein said temperature sensing means sensesambient temperature and oxygen sensor temperature.
 7. The calorimeteraccording to claim 1 wherein said flow meter includes an upperultrasonic transducer and a lower ultrasonic transducer in fluidcommunication with the inhaled and exhaled gases passing through saidflow pathway.
 8. The calorimeter according to claim 7, wherein saidtemperature, pressure and humidity sensing means are microscopictemperature, pressure and humidity transducers arranged in an arraywithin said ultrasonic transducer.
 9. The calorimeter according to claim1, wherein said respiratory parameter is resting metabolic rate, whichis calculated from the measured volume of oxygen consumed by the subjectand the measured amount of carbon dioxide produced by the subject. 10.The calorimeter according to claim 1, further comprising a carbondioxide sensing means for sensing the amount of carbon dioxide in theexhaled gases.
 11. An indirect calorimeter for measuring the metabolicrate of a subject, said calorimeter comprising: a disposable portionhaving a respiratory connector configured to be supported in contactwith the subject so as to pass inhaled and exhaled gases as the subjectbreathes; a flow pathway within said disposable portion operable toreceive and pass inhaled and exhaled gases, said flow pathway having afirst end in fluid communication with said respiratory connector and asecond end in fluid communication with a source and sink for respiratorygases; a reusable portion having a housing with a recess defined thereinfor receiving said disposable portion; a flow meter within said housingconfigured to generate electrical signals as a function of theinstantaneous flow volume of inhaled and exhaled gases passing throughsaid flow pathway, wherein said flow pathway includes an elongated flowtube through which the inhaled and exhaled gases flow, and a chamberdisposed between said flow tube and said first end, said chamber being aconcentric chamber surrounding one end of said flow tube and beingdefined between said flow tube and said outer housing; a temperaturesensing means within said housing operable to generate an electricalsignal representative of ambient temperature; an ambient pressuresensing means within said housing operable to generate an electricalsignal representative of ambient pressure; a humidity sensing meanswithin said housing operable to generate an electrical signalrepresentative of relative humidity; a component gas concentrationsensor within said housing operable to generate an electrical signal asa function of the instantaneous fraction of a predetermined componentgas in the exhaled gases as the gases pass through said flow pathway;and a computation unit operable within said housing to receive saidelectrical signals from said flow meter, said temperature sensor, saidhumidity sensor, said ambient pressure sensor and said concentrationsensor and operative to use said electrical signals in calculating atleast one respiratory parameter for the subject as the subject breathesthrough the calorimeter.
 12. The calorimeter according to claim 11,wherein said respiratory parameter is resting metabolic rate, which iscalculated from the measured volume of oxygen consumed by the subjectand the measured amount of carbon dioxide produced by the subject. 13.The calorimeter according to claim 12 wherein said flow meter includesan upper ultrasonic transducer and a lower ultrasonic transducer influid communication with the inhaled and exhaled gases passing throughsaid flow pathway.
 14. The calorimeter according to claim 13, whereinsaid temperature, pressure and humidity sensing means are microscopictemperature, pressure and humidity transducers arranged in an arraywithin said ultrasonic transducer.
 15. The calorimeter according toclaim 14, wherein said component gas concentration sensor is an oxygensensor.
 16. The calorimeter according to claim 15, wherein said oxygensensor is a fluorescence quench type oxygen sensor.
 17. The calorimeteras set forth in claim 15 wherein said temperature sensing means sensesambient temperature and oxygen sensor temperature.
 18. The calorimeteraccording to claim 15, further comprising a carbon dioxide sensing meansfor sensing the amount of carbon dioxide in the exhaled gases.
 19. Anindirect calorimeter for measuring the metabolic rate of a subject, saidcalorimeter comprising: a disposable portion having a respiratoryconnector configured to be supported in contact with the subject so asto pass inhaled and exhaled gases as the subject breathes; wherein saiddisposable portion includes an outer shell having a ceiling at an upperend, a floor at a lower end, and a rearward wall extending therebetweensaid ceiling and said floor, and said ceiling includes an opening with apredetermined pathogen resistant material disposed across the openingand said floor includes a first opening with another predeterminedpathogen resistant material disposed across the opening, and a secondopening, and said rearward wall includes an opening with anotherpathogen resistant material disposed across the opening; a flow pathwaywithin said disposable portion operable to receive and pass inhaled andexhaled gases, said flow pathway having a first end in fluidcommunication with said respiratory connector and a second end in fluidcommunication with a source and sink for respiratory gases, said flowpathway including a flow tube through which the inhaled and exhaledgases pass, and a chamber disposed between said flow tube and said firstend, said chamber being a concentric chamber surrounding one end of saidflow tube and being defined between said flow tube and said outerhousing; a reusable portion operatively attached to said disposableportion, and having a passageway in fluid communication with the sourceand sink for respiratory gases and in alignment with the second openingin the floor; a flow meter within said disposable portion, wherein saidflow meter includes an upper ultrasonic transducer for measuring theinstantaneous flow volume in alignment with the opening in the ceiling,and a lower ultrasonic transducer for measuring the instantaneous flowvolume in alignment with the first opening in the floor; a temperaturesensing means within said housing operable to generate an electricalsignal representative of ambient temperature; an ambient pressuresensing means within said housing operable to generate an electricalsignal representative of ambient pressure; a humidity sensing meanswithin said housing operable to generate an electrical signalrepresentative of relative humidity; a component gas concentrationsensor within the disposable portion that is in alignment with theopening in the back wall; a computation unit within said disposableportion, wherein said flow meter generates an electrical signal as afunction of the instantaneous flow volume of inhaled and exhaled gasespassing through said flow pathway and said component gas concentrationsensor generates an electrical signal as a function of the instantaneousfraction of a predetermined component gas in the exhaled gases as thegases pass through said flow pathway and said computation unit receivessaid electrical signals from said flow meter, said temperature sensingmeans, said ambient pressure sensing means, said humidity sensing meansand said concentration sensor and calculates at least one respiratoryparameter for the subject as the subject breathes through thecalorimeter.
 20. The calorimeter according to claim 19, wherein saidtemperature, pressure and humidity sensing means are microscopictemperature, pressure and humidity transducers arranged in an arraywithin said ultrasonic transducer.
 21. The calorimeter according toclaim 19, wherein said component gas concentration sensor is an oxygensensor.
 22. The calorimeter as set forth in claim 21 wherein saidtemperature sensing means senses ambient temperature and oxygen sensortemperature.
 23. The calorimeter according to claim 19, furthercomprising a carbon dioxide sensing means for sensing the amount ofcarbon dioxide in the exhaled gases.
 24. The calorimeter according toclaim 19, wherein said respiratory parameter is resting metabolic rate,which is calculated from the measured volume of oxygen consumed by thesubject and the measured amount of carbon dioxide produced by thesubject.